I can’t install my Operating System agai

One day I started my PC and when it turned on, an error message showed up. It said “No Operating System Present”. This appened several times, even when I turned off my PC. I thought that my PC had a virus. So I formated my hard disk, but when I tryed to install the operating system again, (WinMe, Win98, even Win 95), when the Scandisk runs says that there’s a problem with the amount of the Hard Disk and quits. This is the next error message that I have after that:

run-time error R6001
– null pointer assigment.
I already used fdisk to erease and crete my partitions again, but nothing appens. Please, HELP ME.