We had a client whos computer started and indicated that windows found a
problem with the registry and that a good system was restored. Windows
started and froze, after that the drive would not boot. IF you went into
BIOS it detected a drive. When booting from the 98 boot floppy a message
would appear indicating the there was no valid fat32 partition. After
booting from the floppy there would be no C: drive and FDISK showed no
partitions.

FDISK / MBR was run from the boot floppy and everything came back to normal.
We instructed the client to do a virus and spyware scan using the latest
definitions and it came up clean.

This same thing has also happens more than once on the sam computer. We
replaced their RAM and cables the first time and put in a new HD.

What could cause this to happen?

If this was caused by a virus wouldn't it show up in a scan?

If the problem was related to hardware, is it safe to let them continue
using the system as is?

Thanks for any assistance.