Normally I keep a low profile when talking about computers and operating systems since those discussions often devolve into holy wars. But on this occasion I'd like to chime in...
I've been building and fixing PCs since 1985.. Working on Macs since 1997 - I can at least claim to know a thing or three about both platforms (and also several flavors of Unix)
Four years ago I took a good long hard look at everything going on in the Windows world - decided I was not enthused with where things were going and I proceeded to buy a Mac for my daily use.
I transitioned over slowly but by the end of 2003 I had completely switched over and just had a Windows machine here to help service the computers of my computer clients. Since then I've bought three more and my whole family is using them.
I haven't looked back since.. Four machines being used daily and I can count the hours I spend on maintenance in a month on two or three fingers.
I have - had the opportunity on several occasions now to deal with new PCs that shipped with Vista. Ranging from low end to high end machines. My customers are having nothing but problems with their software and with drivers for their hardware.
In my opinion, the XP to Vista transition is and will continue to be even uglier than what Windows 3.11 to Windows 95 was.
Three of my clients actually went out and purchased XP to put on their new machines that shipped with Vista. I have a couple now who are using Intel Macs with Windows XP either booting natively or using virtual machines to ease their transition.
Many of my clients have followed my lead and have purchased Macs in the past two years. I never hear from them any more except when they want a memory upgrade or a bigger hard drive - or want to know what new Mac they should buy. (That part hasn't actually been any good for business.. service calls are money in my pocket..) They are happy, their kids are well adjusted and even their dog likes them again.
My Windows clients are not so happy these days.
Guess my point here is you might just want to keep your eyes on other alternatives - but at least count me in as a vote for the camp that says Stick with XP for now if you are determined that you want Windows. Especially if you have existing software and peripherals you intend to move to the new machine.