COPIED FROM LINK ABOVE:
The CMOS battery is dead or dying
Explanation: The system is exhibiting behavior that implies that the CMOS battery is dead. This can include lost CMOS settings, the real-time clock losing time, or of course dead battery warnings at boot time.
Diagnosis: On an older PC, it is normal for the CMOS battery to fail at some point in time. They usually last for many years, with over five years being the norm, at least on older machines. Nobody knows for sure how newer machines will fare. On a new motherboard, this sort of message is a sign of a defect, although you shouldn't worry about it if it appears only the very first time the board is powered up. The solution is replacing the battery, and this can be an either easy or impossible task, depending on how much thought the motherboard manufacturer put into the design.
Recommendation:..........See INFO in Link above