First, let's dispel a few myths about the latest three versions of Windows:

"Windows XP is outdated".

Oh, really?  Darn, that's terrible news, since I'm still using it and will happily stop using it when they pry it from my cold, dead fingers.  When every program you use works perfectly, doesn't it raise the question, "How do you improve upon perfection?"  It does with me, so I've promised myself that just as soon as my WinXP breaks, I'll upgrade.

Update:  I have now switched to Win7 for the exact same reason I'm usually forced to upgrade:  I was desperate to use a program that only ran on the new OS.  In this case it was Google Earth Pro, the commercial version that makes video tours.  It not only ran much slower on XP, but was subject to lock-ups.  Then I discovered that my Verizon Wireless was also quicker, so between the two of them I capitulated and am now putting up with the deficiencies of Win7 in order to reap the benefits.
 

"Windows Vista is bloated"

Far from it.  Yes, there are a bunch of things running in the background, which might appear as 'bloatware' to the uninitiated, but all of them can be easily turned off.  Once you do that, Vista is a solid, robust operating system and is arguably the best of the three because it's far newer than XP, but doesn't suffer the limitations of Win 7.
 

"Windows 7 is the answer to both of the above!"

Ha ha ha.  The simpletons at Microsoft were so desperate to get rid of Vista's "bloatware" that they not only removed most of the background routines (some of which, you know, might have actually been important), but these jerks also removed a whole bunch of standard little Windows programs like WordPad and Movie Maker, all in an effort to trim things down.  These bozos even removed the email program.

For a whole shitload of busted myths, check the 'Facts & Fallacies' page.

     
As for this site, about 90% of it has nothing to do with the actual version, and I'll color-code the parts that are specific to one OS so you can find your individual tweaks easily.

Standard Disclaimer:  As on any site like this, I completely don't guarantee a single thing here will work, and if your tower turns into a molten fireball of melted plastic and twisted metal from trying one of my suggestions, tough titty.

The most important pages on the site are the Setup pages.  If you do or read nothing else, run through the Setup page for your version.  You really need to learn how to turn off the unnecessary background routines that new programs install.  When it comes to the general operation of your system, that's as important as everything else combined.