It's a full retail license you would need, and if you don't have a retail copy of XP, you can't simply buy a new license from Microsoft and expect it to work.
More than that though, you should make sure there are actually XP drivers for your hardware. You might well be stuck with Vista like it or not, unless you're willing to give Linux a try.
I just got a new pc with vista on it and I already want vista gone. I have xp pro on all of my other computers and that is the way I like it. I cloned the c: drive from one of my computers (xp) and I want to install it in place of the vista drive. When I boot the pc up xp will of course realize that there were major hardware changes. I know that I will have to buy a license key from MS but the question is will I need a full install key or will an upgrade key work (and be legal)? Just to be clear here, I am NOT trying to pirate software, I just don't want to buy the upgrade only to find out it won't work and then have to buy the full install anyway.