ok, here's the thing. xp pro comes in two differnet things. the first one, is the "upgrade" the upgrade is just as the name suggests, an upgrade. if you were to ever format your hard drive, howeve,r you would need the full version of windows to restart sucesfully. actually, to restart period. so, tell me, do you think yuou will ever format your hard drive? of course, the second option is the "rea" version of windows xp pro. this is the full disc. you could format your comp and just pop this in and your comp would read it and start to intsall it. of course, if you use to upgrade disc, i think that it might slow your comp down. i would just buy the full disc, back up your hard drive, and format your comp. then pop the "full" disc in.
both the full version and upgrade version on the same page: