and look near the CMOS battery (it's round,silver-coloured, and the size of a 5p piece) for a small jumper. It's put there so you can clear the CMOS (the onboard memory which stores any changes made to the default BIOS settings). Consult your motherboard manual for the correct location if you can't find it.
To clear the CMOS set the jumper to the closed position (with the jumper bridging both metal pins). Press the power button to turn the computer on. Your system will not power up, but the CMOS will be cleared. Then, open the jumper (put it back to the default position) and power on the computer normally. You will most likely be stopped in the POST screen and prompted to press F1 or another key to set BIOS defaults. Do that, and from the BIOS screen, simply save and exit to start your computer normally, minus the BIOS password.
If you can't find a jumper, remove the CMOS battery and leave it for around five minutes - then replace and start the computer normally.