You, like thousands of others, may have installed vista on a machine with a gig or less of ram, and thought you made a good choice because microsoft's total care advisor may have said it will run smoothly. Well that is just wrong!! If your pc wasn't preloaded with vista, or your xp machine has less than 2gb of good speed ram, a 120gb+ hdd, and a dedicated video card, don't even bother waste your cash! I've done the same mistake once, and vista worked great on a 2005 one gig ram 250gig hdd great for two weeks, and stopped working then ( after the 14 day warranty expired). I was left with a slow piece of junk until i went back to xp mce and it sped up again. I had a 150.00 cd left useless it the cabinet. So what did i do? I sent that piece of junk back where it came from (best buy) (with lots or arguing, i managed to get by!). Then, i got an awesome laptop with two gigs of ram and dual 120 gig hdds, and it worked like a charm (and still is). So if your xp pc has 2gb+ ram, 120gb+5400rpm hdd, dedicated 128mb+ graphics card, then you can smoothly upgrade. Otherwise, stick with xp.
Here is an overview of ram and vista:
1gb vista= 256mb xp
2gb vista= 1gb xp
4gb vista= 3gb xp
I hope that helps you, and i'll post more helpful reviews.
P.S. Remember: if it ain't broke don't fix it!