A checksum is computed as an error-detecting code, to protect the BIOS settings stored in the CMOS memory. Each time the system is booted this number is recomputed and checked against the stored value. If they do not match, an error message is generated to tell you that the CMOS memory contents may have been corrupted and therefore some settings may be wrong. BIOSes react in different ways to encountering this sort of error. Some will warn the user and then continue on with whatever settings were in the CMOS. Others will assume that the settings that were in the CMOS were corrupted and will load default values stored in the BIOS chip "for safety reasons". The error message will indicate which your system is doing.
The most common cause of checksum errors in CMOS is a battery that is losing power. Viruses can also affect CMOS settings, and motherboard problems can also affect the stored values.
First try to reset the BIOS jumper as nobus suggested, if that doesn't work, look here:
http://www.pcguide.com/ts/x/comp/mbsys/bios_Corrupted.htm
