invalid media type reading drive c

Hi,
I have a client who was having problems with the display settings on his computer. He thought if he uninstalled W98 that W95 would just be there(he thought it was an upgrade from W95). He reinstalled W98,and during setup he got the message thatW needed a certain amount of space to install. At that point he called me, I told him to go into fdisk and tell me the partition info. system unknown. I told him to format the drive and try reinstall. Same thing. I had him do a chkdsk and he got theerror"invalid media type reading drive c.
What could be going on here. He does not think he did anything to get a virus. I was going to do fdisk/mbr if he might have a virus.
Thanks