You usually press DEL or one of the F keys while it's doing the POST (Power On Self Test). It should say on the screen too I think.
EDIT: Hmm. Maybe I've misread the message you posted above. The checksum error could be a lot more serious and the prompt for a system disk may actually be for one with a BIOS on it or something, not the normal prompt for a bootable disk.
Looking at this thread someone has the same problem as you but no solution to the problem as far as I can see. Could be the chip the BIOS is on is damaged, or the battery has run out or something. The previous owner didn't try (and fail) flashing the BIOS or something? Maybe something happened there? It might be worth checking that everything is seated properly on the motherboard too. Cards, batteries, connectors, jumpers.. that kind of thing.