I know this is highly illeagal....but can you install service pack 1 with a copied version?
Microsoft came up with a list of "known" pirated keys, and have supposedly blocked the ability to install SP1 if your XP had one of those keys.
I actually don't know what happens that "blocks" you from installing it. Are you still able to use your current XP without SP1? Or do they make that one not work anymore?
Keys DO exist that Microsoft has not banned.
Also, there are several methods to beating the system. Just go to google and search for the appropriate terms and I'm sure you'll instructions.