OK, time for some computer geekery, and a little helpful advice from The Freeholder.
Long-time readers know that I work in the computer industry, doing things I can't mention for a place I can't name. As a part of my work, knowledge of Windows is one of my stocks in trade.
When Windows Vista came out about a year ago, there was what I felt was the predictable wailing and gnashing of teeth. I wrote this off as typical end user resistance to change, with a side order of Microsoft bugs. However, as time has passed and I've had the opportunity to work with Vista, and I've listened to others who have worked with Vista, I've come to a conclusion: Vista is not a worthwhile upgrade. You don't want Vista. Stick to XP, or go buy a Mac.
The thing that really got me into posting mode on this was this piece on News.com where Windows XP was benchmarked as running twice as fast as Vista on the same tasks. (You can get somewhat more gory technical detail here if you need it.)
So here's some free advice from The Freeholder. If you have an older, non-Core 2 duo machine, stick with XP. You can run XP along with a good security suite such as Trend Micro Internet Security on any Pentium 4 with 512 MB of RAM and get acceptable performance for all everyday tasks. (Of course, a faster processor and more RAM won't hurt. I'm running that configuration of a 3.0 GHz P4 with 1.5 GB of RAM, and it hums along pretty quickly.)
If you buy a new machine, depending on the model and the brand, you may get a choice of XP or Vista. If you get a choice, take XP for now, but be sure to get at least 2 GB RAM installed (4 would be better), and be sure that you can upgrade to Vista for free at a later time if things should change for the better with Vista.
If you decide on a machine that only comes with Vista, then get the fastest processor, the most gee-whiz video card and the most RAM (up to 4 GB) that you can afford. Be sure all your peripherals (printers, scanners etc.) are either new or have Vista drivers available from the manufacturer. Be prepared to change or upgrade a lot of your software, and work on your patience. You're going to need it.