I am having problem in saving the CPU speed and mm/dd/year setting in the BIOS. After setting it in the BIOS menu there and pressing F10 to save it. After restart the clock and the CPU show correctly during boot post. When I shutdown the computer and call it a day,after sometimes when I restart the cpu showing my speed is 500MHz. instead of 1GHz and the year back to default 2001. I tried several time but it seem the same. My OS.P3. XP Pro.SP2. Please help me to do it correctly. Thanks.

Your CMOS battery is empty.

How to replace the CMOS battery

My first guess would be flat or weak CMOS battery.