Replacing your CMOS Battery
General Misc.

1. Boot your PC and enter its setup mode. If you watch your screen closely as it boots, it usually will tell you how to enter your Setup. Often it's pressing F1.
2. Write down all of the settings from the various menus.
3. Power off your PC.
4. Open the case and locate the battery on the motherboard. See your user manual for specifications about the battery and its location.
5. Obtain a replacement battery from a local or online computer parts dealer.
6. Remove the old battery and replace it with the new one.
7. Document the date you replaced the battery.>
8. Replace the case and power on the PC.
9. Enter your PC's setup mode.
10. Reenter the settings you have written down from the various setup menus.
    

1. Norton Utilities and similar software suites usually contain utilities that allow you to backup CMOS settings to a floppy. Use such a utility in place of writing down and reentering settings.
2. Don't forget to observe proper anti-static precautions when operating inside the case of your PC.
3. If you can't see your battery right away, try removing expansion cards or unplugging cables. Most newer motherboards use lithium batteries that look like large watch batteries.

This excellent tip courtesy of CNET