Sorry to repeat some things but the suggestions are drifting off target
1. I'm stuck with DOS. Windows setup (from the floppy) locks the machine.
2. Hard drives have been wiped with FDISK and reformatted (showing that FDISK and FORMAT both work). I have now tested both HD's on another computer and there's nowt wrong with them - surface scan with Scandisk reports no bad sectors. Whatever caused them to get corrupted was elsewhere.
3. Memory has been replaced but the problem persists. The glitch must therefore be in the mobo or CPU. I haven't yet tested the memory I took out to see if it's OK but I'm guessing it may be useable.
4. It's a pre USB mobo - Asus P55T2P4 so USB isn't really an issue. Apart from that I've got the thing in pieces here with no devices attached apart from a floppy, one HD and a CD.
5. The CPU isn't overclocked - it's a P200MMX and that's about the max level of the mobo. (On the other hand the machine I'm using here is a P650 overclocked to 850)
So it's now narrowing down. The "event" that caused the problem may have been a spike but it could be a mobo/CPU partial failure. Whatever happened it corrupted the HD's and it still doesn't allow me to go beyond running a few DOS commands.
I'm just about to try disabling the L2 cache as this is an expansion slot on the mobo.