Any copy of Windows can only be activated with Microsoft a certain number of times (3 I believe off the top of my head), after that it'll say it's been registered too many times. It happens to me all the time. But it's not the end of the world, just when it fails, call the toll-free number to register over the phone, go through the motions until it fails again and puts you through to a customer service agent. Just explain to him/her the fact that it's only ever been installed on one machine and whether you obtained your software with the PC or OEM, and she'll issue you a new key for you to key in manually.
It happens to me every time I install a new motherboard.