It honestly doesn’t make much difference to the average user.  The one exception would be if you had a program that only ran on a newer OS, but that’s fairly rare these days.  Most programs are designed to be backwards compatible.

Normally, the obvious recommendation would be to go with the latest, Win7, but the problem is that Win7 is really "Vista Lite", and in their effort to ‘streamline’ things, they threw away a whole pisspot full of classic Windows programs.  Most can still be downloaded from Microsoft, but, overall, it doesn’t make any sense to ‘cripple’ yourself like that just because some fools erroneously claimed Vista was "sluggish" or "bloated" and Microsoft bought into it.

On that subject, yes, there’s a lot of stuff running in the background on Vista, but every single item can be flipped off, turning it into a solid, robust operating system.  Between the two of them, I’d go with Vista and do every little tweak on the ‘Setup Vista’ page.

On the other hand, I, myself, use XP because everything works perfectly and computers, like plumbing, are a classic example of "If it ain’t broke, don’t fix it."  Plus, I like tweaking things, and Vista eliminated a lot of the basic Windows options, and Win7 is even worse.

Summation:

• If you like having as much control over the OS as possible, go with XP.
   
• If you want less control but the full range of Windows programs, go with Vista.
   
• If you don’t intend to do much besides surf and email, go with Win7.