After Nvidia driver install, get 'windows not genuine'

This happened relatively out of the blue:

i was trying to get drivers for my printer. After restart, screen went black. I restarted in safe mode with netwking to find out what to do. So I eventually found out my graphics card had no driver; it had a generic vga adapter. So I search for my graphics card to find it is an Nvidia GeForce fx 5200 and I get the drivers for it. I restart in normal mode: no success. restart in safe' with net' . starts and am notified that drivers have installed. Now on next restart in normal mode, application error 0xc150002...press ok. Then i get a message in the bottom right hand corner that says 'windows is not genuine build 7600'...
I have seen how to change it with/without cmd, but either requires normal mode.

So how do I re enter my product key?

Tews said:

First, do a sfc /scannow to check the integrity of your system files... failing that, do a system repair with your installation DVD..Hope this helps..

I just did that and apparently some files are corrupt. So I guess I have to do a system restore until I get a restore point without the problem?

Thanks for you quick reply.