I have had Windows 7's System Restore brick my PC twice now!
Luckily, I had the OS disc to "Undo" the restore. If you don't have this disc you will want to make a recovery boot disc. Go the "Control Panel" and "Backup and Restore", then on the right "Creat a System Repair Disc".
One thing I have noticed (and it is unnerving)...I needed to "Restart" again after the "Undo" process.
I have used System Restore in XP many times without a hitch...so much for a better OS!