Sorry if I'm repeating what others have said but I would install the "Corporate Edition" of WinXP which does not require activation. Before you do, though, use the Key Gen to make a product key (select Windows XP Corp from the list) and then you'll have no trouble at all. You can even use WindowsUpdate and upgrade to Service Pack 1 with no hassles.

Basically, the original Corporate key (FCK....) has been blocked by M$, so as long as you don't use that one you're sorted