I recently had the same problem when I replaced the motherboard and processor. If you think about it windows would get mixed up with such a large change.
I went down the route of using the xp disc to do a repair. When windows reloaded it would not start until I phoned in to activate it. I had no problem with this but a lot of problems still existed.
I bit the bullet and formatted the hard drive and did a clean install and touchwood has been working perfect ever since. I was then able to activate xp on the Internet this time.