Check the motherboard manual, there is a 3 pin jumper which you can clear CMOS...normally close to the onboard battery.
1. Shutdown computer and unplug power cord;
2. Move the jumper cap to another position, e.g. originally was 1-2, you need connect 2-3;
3. Wait for couple of seconds;
4. Move the jumper cap back to its original position;
5. Plug in power cord, start up computer, press DEL or other KEY designated to enter CMOS setting...