Explorer Error

I had just sold a system with win98.and the enduser came back to me with the system and told me that he tryied to load aol 5.0 from a cd on to the system.now he is getting an illegal opp. error at boot up…it just gets past the logon screen and theerror comes up.it says that the explorer is no longer present and it will not let me into the system even in safe mode…what do i do…i already tryied to format and fdisk the drive but that does not work…and one more thing to add is that this hdd is a 10.4 gb so i had to install a ez-install program to the drive for the board to read the drive…