If Tigsounds advice does not work because you do not know the password then you must reset the BIOS, but you will also lose any custom settings you have. To reset the BIOS you must open up the PC and do one of the following:
1. Move the BIOS reset jumper to erase then put it back to the original position. Download a service/upgrade manual to find the jumper if it not clearly labeled on the mother board.
2. Or unplug the PC and remove the battery from the motherboard for 5-10 minutes.
When you power up your PC you will need to go into the BIOS and reset any custom settings. This will also erase plug-and-play settings, so do not be surprised if Windows finds new hardware on the first boot.