There's two ways I know of doing this. Warning: Both methods wipe all of your BIOS settings back to defaults.
1) Some motherboards have a switch or jumper that you can set to clear the BIOS settings. Check your motherboard manual to see if this is true for you.
2) Take the top off your PC. Somewhere on your motherboard will be a battery. This may be a flat disc, watch-type battery, or it may be a small white battery pack.
If it's a flat disc type battery, remove it. If it's the white battery pack type, unclip it's connection from the motherboard. (The ones I have seen have had a pair of twisted black and red wires connecting the battery to the motherboard.)
Next, switch on the PC and go to the BIOS prompt. Get to a safe point and power the PC back off. Go back into the motherboard and replace the battery, and then power on your PC. Go back into the BIOS and reset things like the hardware date and time. If your BIOS supports it, take a copy of the BIOS to a floppy disk. Do this before you enter any passwords!
Either method requires you to dig inside your PC. The usual warnings apply. ie, you do this at your own risk, and take anti-static precautions. Don't attempt to do anything unless you are confident in carrying out these procedures.
Good luck!
