ok, the cd key on the back is a OEM key it only works on Acer XP home versions, if you are using a retail copy of XP home, it's not gonna work, microsoft has differen keys for different versions of windows.. i.e XP Home, XP SP1, XP SP2, XP OEM, XP OEM SP1, XP OEM SP2, XP PRO, XP PRO SP1, XP PRO SP2, XP PRO OEM, XP OEM SP1, XP OEM SP2....but yea..lol that's why once u format and delete windows xp, fromur PQSERVICE parition, the possibility of find a acer oem copy to reinstall is a lil hard if it is not provided by the manufacturer, best bet, buy a copy of XP, so you always have it, and if you are trying to use windows xp on 2 different systems, you can always just install it, i don't know if it will let u do updates, but you can call microsoft and "tell" them you had it installed on another system but formatted it, removed it from that system and have it installed on a different system, again, not sure if u can get updates on the other one u used the key with...microsoft have become annoying with there "attempts" to stop piracy, and things will only get more complicated when vista is released and they drop volume license keys and beef up the Genuine advantage pop-ups, and other tactics, but yea, as far as loading with a used key and trying it, go for it, if you can switch it once it is loaded...not sure, u might get locked out, but worth a shot, but would kinda suck to get your system all the way you like it then get a update that locks up you computer because you key is for a different version of xp