There is a readily available freeware utility called
'KILLCMOS' on the net which claims to be able to reset the CMOS back to factory/default settings. You need to be able to boot the system up into a DOS environment though.
WARNING - This prog is only recommended for experienced users; it may void the warranty on your motherboard and may cause adverse affects. This is only to be used if all other options of fixing a system have been explored.