If the computer's good, go for Vista, it does actually work except for old stuff (which is fair enough if you think about it, can't really go further into the computing world if you're making sure everything works with 56k Modems..)
I'd also like to randomly note that Vista is actually as fast, if not faster than XP (and it wins hands down in the log on category ;)), and Vista had a better performance at launch compared to XP, which didnt pick up until SP2.
Also, Vista SP1 is coming out soon, which, from what I've heard (i can get it, too lazy :P) does improve Vista a lot (but as Microsoft keep saying won't add features like XP SP2, as that's not what a SP is...)
If you are gaming, Vista is a good bet, as Microsoft as the evil company they are, and making newer games require Vista, and, most games will work on Vista - I was playing Doom on it the other day and that shouldn't work on XP.
If i sound very pro-vista thats because I read all this doesn't work with programs or hardware stuff that I never found true, except it won't work with an old Lexmark printer it works with all hardware and software I used.