Need to get this right, the first time.

[RedDevil]

Almost ALL new PCs are coming with VISTA, If you find anyone still selling XP installed you'll be lucky. The marketing ploy of MS appears to be you sell VISTA or you sell PCs without an OS.
If you have an old XP CD (make sure you have the license code) you can use it to Install the OS on your new PC, you will just have to remove it from your other PC and possibly call MS up to activate it again.

There is trick to getting the NEW PC to come without any OS on it other wise you'll be fighting the install process as Vista doesn't like to be bumped off for XP backgrades.
Depends on where you buy it from, If you're looking at Dell, do a voice order and make sure you get a discount for NO OS installed on the PC
(they like to charge the same price, with or without)

I fought Vista for 2 weeks and Dell sent me XP for free after I asked for return labels to ship them back the PC.