daktaklakpak pretty much mentioned what most systems allow you to do if you managed to lock yourself out of your BIOS/CMOS.
Simply:
Makesure your power is off and cable is disconnected from your PC.
Take the cover off.
Hold with one hand the cage (the metal surface) around your drives or your power supply while working with your boards (Any static charges should be diverted without ruining your board or chips)
You should find a Jumper (Some pin with a little plastic "bridge") that can be moved across. Some systems then use this to bleed the energy from your CMOS, while others will need you to BOOT once in that position.
Then you should place your jumper back to it's original position and replace the cover. and presto, your system should now have a clean CMOS.
Some systems (Laptops) you might have to remove the batterie while others might need you to find the BIOS/CMOS FLASH chip and EXTRACT it. (This will cause your CMOS to "FAULT" and need reconfiguring, perhaps even REFLASHING).
If your really lucky you just might need to FLASH your CMOS (Upgrade its program from your Hardware Vendor or through searching online). Upgrading your CMOS can have advantages, like supporting larger Harddrives, higher processors and other functions that the manufacturer might have only had programmed in Beta originally.
There are also Hack diskettes that you can create from some dodgy sites around the internet that can Bypass CMOS/BIOS passwords that lock you out from full boots. (as some of the other BIOS/CMOS tend to neglect locking you out from Boot disks)