On the mobo there is a jumper switch, usually called "clear cmos" or similar. With the computer UNPLUGGED, simply set the jumper to the "clear" position, then put it back where it was. It will clear any changes you made to the CMOS and revert back to defaults. Then, AFTER you put the jumper back in it's original position, plug the computer back in and fire it up. The only drawback is you'll have to go back in to the BIOS and re-set any custom settings you had.
Hope this helps.
Cheers,
Desciple