Windows date/time clock doesn't work

Is there a battery or something in the computor, that keeps the time set, when the computor is off. I ask this because it doesn't keep time. Every time I turn on the computor on the time is wrong. Can someone enlighten me on this subject.
TNX

 

Thanks Nick

What do I look for? Is it on the Mother Board, Is it like a watch battery or like a AAA battery, or something??
TNX

 

You might want to take a look at this link http://www.hardwaresecrets.com/article/81 I think that will probable answer most of your questions

 

It most likely looks something like the one below. Just copy down the number on the battery and you should be able to find a replacement at Radio Shack.

 

Thank You : All Your Input Was Helpful

Got it done, went to Walgreens and pick up one.
TNX to all who replyed. It was just like in the picture.