Although you don't mention whether you've got a laptop or desktop, the CMOS battery is similar in both. (It's cheap, about $5, and is the size of a nickel.) See the link below for pictures on how to replace it. The BIOS will be reset back to default settings upon replacement of the CMOS battery, so if you've made any changes to the BIOS settings, be sure to write them down before you remove the old battery.
Youtube Video Explaining CMOS Battery Replacement in Desktop
http://www.youtube.com/watch?v=ao866I9XpTo
Or, if you've made BIOS changes that need to be written down, this link will help also.
http://www.computerhope.com/issues/ch000239.htm
Hope this helps.
Grif
My laptop will not hold the correct time, presumably as the batteries are dead. So what powers the clock on my desktop,is there a small feed from the mains even when it is off?

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