download ultimate boot CD. (Google it). Run the tests. (The hard part will be getting a bootable floppy created, hope you have more than 1 PC with a FDD.)
Figure out who manufactured the hard drive. Go to their site. Download diagnostic.
At a guess the circa 1985? hard drive has a few bad spots and you are hitting one before the system has booted enough to handle.
Do you still have other boot media?