To repair a MBR load you XP CD into the drive and restart. Go to the recovery console and type in fixmbr. That is the command for fix master boot record. Might want to double check that. I am almost positive that is it. I havent had to do that is some time now.
I have left my PC on for almost a year now. I only restart when necessary. Aside from that, or the ocassional power outage, my PC runs 24/7. No problems or errors or anything.