|Sounds like the battery on your motherboard that holds your custom CMOS settings has failed. Look for a silver coin shaped battery mount to the motherboard. You can buy a replacement most anywhere batteries are sold. |
Unplug the computer before removing it. Pay attention to which side was up. Ground yourself before entering the case.
You will need to enter the BIOS (setup) after replacement to reset some of the values. Especially, date and time. Possibly the order the computer uses to boot from the devices like the CD drive and hard drive.