System will not boot after install (even from disc or in safe mode)

[kalais]

I have just done a fresh install of Windows xp with SP2 on a friends machine. Running AVG, Windows firewall and nothing else. I installed Dtools lite on it then rebooted.
Result: It will not detect the systems internal drives. It will not even boot from a windows disc...
I have looked through the bios set up and it seems that its not recognising the HD or CD-rom...
HELP! I'm lost!
I will try swapping out the ram (although we just installed 1024mb of working/tested ram) and try another HD too. Any other suggestions? Thanks

[Copytrooper]

If the devices are not detected on BIOS level, neither Daemon Tools, nor SPTD can be the cause. Check PSU, cables, BIOS settings, etc.

[athlon]

I got about the same when I was going to reboot the computer. Disk not detected at boot. When I booted with another disk the "crashed" disk was visible in Windows as a slave. Copied the data and reinstalled the non-booting disk and it works like a charm.