This deals with backing up individual files (pics, text, etc), not your Windows system.

True "backing up" means two distinct drives, not just partitions.  Yes, backing up the stuff on partition D to partition E will save everything if D blows out, but it won't save a damn thing if the drive, itself, goes kaput.

And that's a lot more likely.  Yeah, the C Drive (Windows) will melt down if you look at it cross-eyed, but storage drives very rarely actually blow out to the point where the data can't be read and copied to a safe place.  Hard drives, however, suddenly not doing anything when you turn on the power in the morning isn't uncommon at all.

With that said, if you honestly can't rustle up a second drive, then back things up to a separate partition.  Better that than nothing.  Most systems have a D partition, so make a 'Backup' folder on it, then a bunch of sub-folders for your various files.

To note is the following:

If you have everything on the C Drive, then you should skip this step and start using an image file backup system like True Image.  That'll save both your own stuff and Windows, itself.  Should you accidentally delete a file, you just go into the saved image file and pluck it out.  Should the entire system melt down, five minutes later you'll be back up and running.

If you (properly) only have Windows on the C Drive and your own files spread out all over hell and gone, then do the True Image routine for the system drive and this procedure for your files.
 

SyncToy

This is a Microsoft freebie, available here.  The download button's down to the lower-right.  You want the second (x86) version unless you have a 64-bit system (which you'll know if you do).

Disclaimer: This is fairly straightforward program.  It works fine for me, but if you want something more sophisticated, there are zillions of them out there.  Backup programs are a classic Junior Programmer's pet project.  Do a search for "backup program sourceforge" for a good free version.
 

The Backup Drive

Before you pick up a hot new terabyte backup drive, you need to ascertain if your computer accepts the new SATA hard drives, or still uses the old IDE technology.  If it's newer than 3 or 4 years, it probably accepts SATA drives, but you should double-check.

Pop open the tower and look.  If the present hard drive has a wide, flat, probably-gray cable going to the back, it's IDE.  If it has two normal-looking wires, one of them possibly red, it's SATA.

If it's a SATA drive, follow the (red?) wire to the motherboard.  Make sure there's an identical, empty slot next to where it plugs in, ready for a second drive.

If it's an IDE drive, do the same with the wide cable.  The existing cable probably has both of its "daisy-chain" jacks in use; one to the hard drive and one to the ROM drive, so you'll need a second (empty) slot on the motherboard for the backup drive.

Important Note:  I would have bet a million dollars that my Compaq's motherboard had two IDE slots — until the day I actually tried to plug a cable into the spare slot.  While it looked identical from the outside, it was just a wee bit smaller.  What it went to, I have no idea, but I ended up buying a PCI card with some IDE slots on it.  So you might want to (gently) unplug the existing IDE cable and make sure it plugs into the second slot.

If two IDE cables are already plugged into the motherboard and three of the wide jacks are in use, you can use the fourth, but you'll need to make sure the little jumper block on the back of the two units (sharing the same cable) are set to 'Master' on one unit and 'Slave' on the other.

Also, before you head to the store, make sure you have a second power lead and that it's long enough.  The drive will come with the cable that goes to the motherboard but power leads are our department.  Look at the existing drive to see what it looks like.  SATA drives use a wide, thin black plug, IDE drives use a thicker white plug.  If you're adding a SATA drive and don't see a spare SATA power plug, but you've got a spare white plug, buy a converter wire while you're at the store.  It'll have a white female IDE-type jack on one end and the thin, black SATA plug on the other.

On the subject, sometimes the two power jacks are connected real close to each other (assuming the drives will be stacked close together in the rack), so pre-plan where you're going to put the second drive.  You might need to pick up an 'extension cord' for the power lead, or use an IDE plug with a converter.

As to which drive to buy, bear in mind the 'Which HD?' page.  Fifty bucks can make a big difference.  Assuming your present drive is big enough for a storage drive, you might want to take the opportunity to pick up an ultra-fast SATA drive (see the page) and use it as your new system drive.
 

Hardware Hookup

Stick the cable into the motherboard (both SATA and IDE are polarized) and the back of the hard drive, then plug in the power lead. 

For the placement of the new drive, stick it wherever you want, at any angle you want.  If you can slide it into the little rack, great.  If not, just lay a couple of non-conductive somethings (plastic forks or knives?) across the bottom of the tower and lay the drive across them, raised a bit for ventilation.  I have the front and back edge of my system drive sitting across two blocks of hard foam to reduce vibration and noise and the backup drive stuck in the rack.
 

Getting Organized

You'll want to look over your personal files and decide how you're going to categorize them on the backup drive.  For the most part, you'll just mirror the original folders (if not an entire partition), but for ultra-large folders, or folders that have large (new) files that don't require backup, you might want to fine-tune things a bit when making SyncToy's 'presets'.

Go ahead and make your backup folders, then copy the originals to them.  They'll copy a lot faster straight than through a program.  Then you might add or change a few files in the original folders just to test out your new backup routine.
 

Backup

Fire up SyncToy.  Click on 'Create New Folder Pair'.  Browse to the original folder (or whole partition) on the left, the backup folder on the right, Next.

You can look over the three options but 'Echo' seems like the most logical choice for just backing up files and making sure renames and such get taken care of.  Put in a name for the process and the window will close.

Click on 'Change Options:

• Check 'Exclude hidden files'.  That'll keep it from copying the Recycle Bin folder, the 'System Volume Information' folder, etc.
   
• If you only want to include a few files, enter the names in the 'Include' box, and the asterisk works as a wildcard; i.e., "cool*" would include any file starting with "cool".  Ditto excluding files.  If you want to include or exclude an entire folder, click on 'Select subfolder'.
   
• If there are a lot of large files to delete on the backup drive and you know you won't want to access them, check the 'Save overwritten…' box so it won't take the time to write them to the C Drive's Recycle Bin folder.

To back up your files, just highlight the name on the left and click 'Run' on the lower-right.  A few clicks and you're done.

Note: Occasionally SyncToy stops for a bit to do a big comparative routine.  The more files it's scanning, the longer the pause will take.  Just wait it out.
 

Checking Copied Files

As I disclaimed earlier, this is a fairly simple program and doesn't list the updated files.  Normally, this doesn't make any dif, but if suddenly you want to know what files were just copied, it's easy enough to check.

Open Windows Search.  What you want to do is search your whole backup drive or folder for:

*.*

Asterisk-dot-asterisk tells the computer "look for everything".  In theory, it should list out every file in the backup area.  Then it's just a matter of listing them by date and looking at the most 'recently-modified' files.
 

Isolating The Backup Drive

I have my backup drive on a power switch so it normally stays off, just in case some horrific new virus sweeps through the system, wiping out everything in its path.  In lieu of a switch, and assuming the tower is easy to access, you could just literally pull the plug on the drive when it's not in use.

Here's how to put in a switch:

If it's an IDE drive, the power wire will have a white plug and four wires leading to it, easy to access.  SATA power cords are more complicated, so I'd recommend you buy an 'extension cord' for the IDE type of power plug.  It'll be about 8 inches long and have a white male and female jack on the ends.  That's the piece you'll cut into for the switch.  Then, if the new drive is SATA, buy a converter piece that converts from the IDE plug type to SATA.

You're going to cut into two power wires (5v for the drive's electronics and 12v for the motor), so you need what's called a "double-pole" switch; one that has two separate 'in' and 'out' contacts so it can turn two different wires on and off at the same time.  I think Radio Shack still carries them in the store, or try any electronics outlet.

Regardless of what type of switch you pick up, it should say "DPST" or "DPDT" on the package, with the "DP" part standing for "double pole", meaning it controls two independent wires. You also want to make sure it's just a 2-position switch, on-off, not a 3-position switch, on-off-on.

You'll also need a few feet of wire to lead out to the switch and back.  Anything will do, even speaker wire.  These are low-amperage lines.

Grab the IDE power lead and cut the yellow and red power wires.  Strip back the ends and attach the extra wire to them.  You can use solder, wire nuts or crimp connectors if you want, but just twisting the wires together and wrapping things up with some electrical tape is good enough.

Sneak the wires out the back of the tower and attach the leads to the switch.  Some switches have tiny connection points and you'll probably need to solder them.  Some switches, like the above, use slide-on 'spade' terminals, so you'd need to pick some up and use pliers to crimp them onto the wires, then they'll easily slide onto the switch.

I should probably note that you'll blow up the hard drive if you cross any of the wires, so make sure you keep the red and yellow wires separate, and make sure you know which terminals on the switch to attach the wires to.  If you have an ohmmeter, double-check the switch's connection points.

If looks are important, you can buy a plastic 'experimenter's box' at the Shack to house the switch (good electronics outlets will also have metal ones), then use double-sided 'mirror tape' to stick the box to the side of the tower.  If looks aren't important, just stick the switch to the tower. 

And that should do it.  Firing up the backup drive will require a reboot, and I'd suggest you actually shut down the system, flip on the backup drive, then turn everything back on.  Ditto turning it back off.  Both Windows and the hardware end of things (like the power supply) deserve some respect in this department.  You can plug and unplug removable devices all day long, but you don't want to mess around turning hard drives on and off while the system is up.