Thank you very much for the responses.
I've managed to get it sorted now, it seems, by flashing the BIOS and removing some of the RAM.
Just in case anyone else encounters the same problem, this is what I did:
-Used another PC to download a BIOS update.
-Put DrDOS and that BIOS update on a floppy.
-Made a boot CD from the floppy using Nero.
-Booted the new machine with the CD and flashed the BIOS.
-Removed half my RAM (was getting BSODs when installing vista).
Hope Vista will prove worth the hassle...
Thanks again for the advice!