I run across this problem a few times now.
It's a bit tricky!Some people just reinstall. I don't!
I have to ask: Do you know to use the comand prompt?
I resolved this issue with the installation cd, by doing a repair with the recovery console.
The tricky part is that you need to manually use comand prompt to backup a file, then copy the same file from the Repair folder to System32/config folder.