You'll laugh but in the past I encountered similar problems several times
when trying to install Win2k from a CDRW drive. Substituted a standard CDR
drive and all was fine. Don't ask why.

On getting to the HDD being the only device that I'm able to replace, I
noticed the possible problem!!

One of the little high speed ATA cable wires had got shredded against the
metal work and was hanging off. Fingers crossed. Either it's always been
broken or was broken during many of the removals of RAM, HDD, CD-ROM whilst
trying to get it working.

I hope the former...

Cheers, Rob.