Windows 7 System Builder license terms:
http://oem.microsoft.com/downloads/P...se_English.pdf
One feature is that it is intended only for resale; you're not supposed to install it on a system for personal use. (As far as I know, it's not enforced.)
As for other details, see the FAQ:
Licensing FAQ
Microsoft may choose to allow activation on a new motherboard if they regard it as a repair, replacing a defective motherboard. It's up to their discretion, though. (From what I've read, MS has activated repair installations on hardware that was quite different from the original.)