CMOS Battery

The CMOS (complementary metal oxide semiconductor) chip is a tiny chip on the motherboard of your computer that holds a small bit of code which helps your computer boot up to Windows.

Your computer accesses this code when the power is switched on. The code tells the computer where to find the disk from which it should boot up. This disk is usually the hard drive. The chip also holds the current date and time setting for the computer.

If your computer starts losing its date and time settings, you can be pretty sure your CMOS battery is running out of juice. If you don't change it out soon after, you will eventually get to a point where the computer cannot find its own hard drive.

Windows will start to load, and then the computer will reboot and try to start Windows again. You may see a blue screen flash up with an error that says "Unmountable_boot_volume".

If this happens, you'll need to first replace the CMOS battery. Most of these batteries look like large watch batteries. They are usually about the size of a quarter and might have a number like CR-2032 etched into them. Below are the steps to find out which battery your PC is using and how to replace it.

NOTE: The following steps are for your information, and you understand that I am not responsible for the results of any work you do on your computer. Having said that, please know that replacing a CMOS battery is very easy.

You can verify the part number of your computer's battery using these steps:

1. First, unplug the computer.
2. Open the computer case. (You'll want to ground yourself to something metal by holding onto it after you open the case, to discharge any static electricity).
3. Look on the green motherboard for a silver circle with writing etched into the top. That's your CMOS battery.
4. Get the number off the top and write it down.
5. Go to a computer store or office supply store and find a battery with the same number as the one in your PC.

Now if you are feeling brave (it's really not that hard) you can replace the old battery with the new one. Again, ground yourself after you open the case.

1. Using the edge of a butter knife or a binder clip, gently pry the old battery out of it's black "case". Then snap in the new one. Voila, you are done.
2. Close your computer case. Hook it all back up again.

After you put the new battery in, boot your computer. If Windows loads, and everything looks fine, you are done.

Still Won't Boot?

However, if you still get the Unmountable volume error, you'll need your original Windows XP CD that came with the machine.

1. While the machine is on, put the Windows XP CD in the CD drive, close the drive, then shut down the machine completely. Let is sit in the off state for about 20 seconds.
2. Turn the machine back on. It should ask you to boot from the CD. (If it does not, you'll have to go into the Setup and set the order of the boot devices to make the CD first. You can usually get to the setup screen by pressing F2 when the machine first starts.)
3. Boot the computer from the CD. It will say that is is checking and loading Windows. When it gets to a question screen, choose Recovery mode by pressing R.
4. It will run for about 2 minutes and then give you a C:\ prompt.
5. At the C prompt, type chkdsk /r and press enter
6. The computer will begin checking its hard disk for errors. Let it completely finish. Then reboot.
7. When it is rebooting, take the disk out of the CD drive, so that it will boot from the hard drive instead of the CD.
8. It should boot into Windows now.

Done with CMOS, back to Computer Questions