as above,
If you just change the cpu, then No, you dont need to reinstall windows.
Changing the mobo as well, then Yes you will have to reinstall windows.
Also, if its an OEM version of windows, then that will be tied to the old mobo, and will NOT activate on the new one, as Microsoft class a new mobo as a different pc.
People have been known to contact Microsoft with a sob story and getting it to activate, but there is no guarantee.![]()
It's a bit of a lottery. I've been lucky so far, just had to phone up automated phone system to get activated. Just did a mobo swap on a Packard bell OEM Vista Basic.