I can't boot and can't get into the BIOS because my keyboard is not recognised. I have flushed the BIOS. I get two error messages: 1. checksum error defaults loaded; and 2. Warning CPU has been changed. Then the system hangs. Does this indicate a bad cpu? Or perhaps bad BIOS?
A checksum error generally means that the file/data being checked at startup is different than a previous (good) startup. The CPU warning could mean either that the chip isn't being read correctly (could be loose).
Basically these two warnings together indicate that the problem lies with the CPU. If the CPU is being read correctly, then BIOS is seeing that the data is different than a previous POST. Have you changed the CPU (upgrade, downgrade, same grade)? If you haven't then your CPU could be failing. I'd remove the heatsink and inspect the CPU chip.
As for your keyboard, if you're using a USB keybaord, it's possible that your BIOS doesn't recognize USB. Either switch to a PS/2 or use USB to PS/2 adapter.
I did two things: bought a new keyboard and reattached the heat-sink with new thermal paste. The key board was not recognised using the usb port but I used the PS/2 adaptor and I was able to get into the BIOS. I don't know which of the two actions did the trick. For good measure, I am going to reinstall WinXP because the boot-up was very slow.