Well, you've eliminated/changed 1 variable, you might want to try others. Unplug the data cable to the hard drive and boot to a (Win98) floppy disk. See what happens there. Plug the drive back in, boot to the floppy and see if you can read the C: drive, type dir c: at the floppy prompt.
If the drive can't be read, temporarily plug it into the Secondary IDE channel and try again. Could be the IDE controller on the motherboard is flaky/bad.
Obviously you have partitioned and formatted the C drive so if you can read it, then it could be problems with the memory.
If you 2 or more memory modules try taking one at a time out and see if that helps. Be sure the single module is in the first slot.
From a distance, it's hard to advise specific actions. It's just a remove/replace process when it can't be seen on a tech's bench.
Remove all the plug-in cards except the video and try it. Then replace one until you get the problem again.