With the PC turned off remove the BIOS battery (disc battery usually near bottom of MoBo) and short the two parts of the battery holder together for a couple of seconds. This will clear the BIOS. Re-insert the battery and turn the PC on, you will now need to re-configure the BIOS for your hardware. If you don't know how to configure the BIOS for the hardware - don't do any of this, take it to someone that can.