Aqua-Soft ForumsI haven't upgraded my computer in a while and I can't remember if I need to reinstall XP after a simple CPU upgrade or not. Not the MoBo, just the CPU. Anyone know for sure?
[help]cpu Upgrades And Xp
technically, no, you don't, as long as your mobo isnt touched.
BUT, XP has "activation" and if it finds itself running on a "different" PC, it will disable itself. Ive heard of problems where simply changing the CPU makes XP think its on a different machine.
So the answer is, "only one way to find out". 
Well, YMMV, but XP Pro gave me no troubles when I upgraded my BIOS, swapped a Duron 1.3GHz for an Athlon XP 2400+, and swapped my network card.