On a Windows XP Home system that wouldn't boot and gave an error that said "Error loading operating system", I ran
fixboot from the XP install disk. It seemed to successfully fix the boot sector after reporting that it was corrupted.
I then exited from the Recovery Console and ejected the disk. The system booted successfully into Windows. After shutting down and restarting, the error returned. I've run a virus scan with a recently updated AVG.
Reading the S.M.A.R.T. data on the drive, and running the quick diagnostic shows no errors.
Any ideas on what may be causing this and what to do about it?