if it was 3 months ago, you will have no problems reactivating on the new system.
If it was shorter than 3 months... Ring up Microsoft (they'll give you the toll free number when you choose activate by phone) and explain to them. Despite what people seem to say, i didn't get any major problems. I rang, they did a quick check, reset my activation and voila... Activated XP =)
Explain clearly that this is a new system that you have purchased and that Windows XP is no longer the OS on the old system. If they ask which OS just say Linux. (Unless you have a legit Win98)
WinXP is the only OS i have purchased =P... The other ones just weren't worth it... not to mention every joe bloggs had a copy somewhere in their CD stash.
Ep, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS