All Windows PCs contain a small battery that powers a chip, which in turn stores important system data called the CMOS settings. If that battery runs down, when you attempt to start up the computer you'll get errors such as "Invalid system settings--Run Setup" or "CMOS checksum error." If you aren't ready to buy a new PC, it's time to put in a new battery.
Instructions
Restore the CMOS settings
1. If you've never made a backup copy of your CMOS settings (by using a backup utility program such as Norton Utilities or by printing a hardcopy out on paper), look them up in the printed documentation for your system or call the vendor of your PC. Restoring your CMOS settings from a backup will help determine whether the battery is the problem.
2. If you're using a backup utility to restore the CMOS settings, insert the emergency startup disk and follow the prompts.
3. Use the Setup utility that's built into your PC's BIOS to re-enter your CMOS settings manually. Restart your computer and wait for a screen that tells you what key or key combination to press for Setup.
4. If re-entering or restoring your CMOS settings solves your problem, then it's possible your battery is fine and the settings were corrupted by a virus or some other anomaly. If, however, your PC "forgets" the CMOS settings you just re-entered or restored after you turn the machine off (and then back on), you probably have a bad battery.
Locate the dead battery
5. Turn off your PC.
6. Before opening your PC case, put on a grounding wrist strap to prevent discharging static electricity onto any sensitive components. In fact, throughout this procedure it's a good idea to frequently touch something metal (other than your PC) that's resting on the ground, to make sure you discharge any static electricity.
7. Open the PC. For most PCs, this entails removing a few screws with a Phillips screwdriver and sliding the case off.
8. Locate the battery on your PC's motherboard. This is trickier than it sounds because PC manufacturers have used many different types of batteries for CMOS settings. The most common are lithium, like the kind in watches, but they could also be a pair of AA batteries. Or they could look like two cylinders encased in red plastic: a silver box or a red and black box.
9. Draw a picture of the battery, showing its exact position on the motherboard.
Remove and replace the battery
10. Examine the battery carefully to see how it's attached to the motherboard. Most likely, the battery is attached by a clip or with Velcro. Some older PCs might have the battery soldered to the motherboard. Unless you're confident with a soldering iron, don't attempt to replace one of these.
11. After you've removed the battery, take it to an electronics store to match it with a replacement.
12. Replace the new battery in exactly the same position as the old one, referring to your drawing (see Warning).
13. Restart your PC and re-enter or restore the CMOS settings.
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