|Sounds like the CMOS battery is too weak to hold the settings. The CMOS battery is a quarter sized flat silver battery mounted to the motherboard. You can buy them most anywhere batteries are sold. |
Unplug the computer before entering the case and take anti-static precautions. Also pay attention to the orientation (which side up) of the battery before removal.
Upon first boot you will need to immediately enter the BIOS screens to reset values as required. Date, time, boot order, possibly others too. Removal of power to the board resets all CMOS values to defaults.