hi everyone

i was working on a friends computer last night, randomly when he boots it he gets an "operating system not found" error. after messing with it for a little bit i got it to load windows, and i tried to defrag his hard drive but i got some strange errors. so i tried to format the hard drive, and a minute or two into the format it would tell me error, hard drive is changing.

i don't know what would make the hdd change during a defrag or format, could that be a virus? i suspect an mbr virus since it doesn't boot correctly occasionally. thanks in advance for the help!