Before doing any hardware work to your computer it is best to backup your hard drive, if you are able to do that.

The CMOS battery is located on the motherboard. It's a little silver, round, flat battery which kind of resembles a watch battery, although the CMOS batter is just a bit bigger then a nickle, but smaller then a quarter. After your computer is turned off AND UNPLUGGED (important!), open up the box, ground your hands by touching something metal before reaching your hands into the computer (but not on the box itself...something else that's metal), and look for that little, flat battery. Sometimes it's in there pretty good, and you need a chip puller tool to get it out, but if you are careful you may be able to use your fingernails. It's usually pretty easy to get out, and it does not require any need to be afraid. Just use caution not to touch the motherboard or scratch it or anything. Once it is out, make your trip to the supply store as quick as possible. I forget how long it is OK to remove the battery from the mainboard, but I think it's a couple of hours before some problems may occur. Then you just take that little battery to your nearest computer supply store and if you can't find the correct voltage, get a store worker. Just make sure the voltage on the new battery is the exact same as the old one. The voltage will be engraved in the top of it. Once you have the new one, ground yourself again before reaching into the box by touching something metal and pop that little battery in.

It's now safe to plug in and turn on your pc. Your computer may start running the setup mode, if you are not comfortable with the setup mode then look down at the bottom of the setup and you will see different keys to press to do different things. Look for the key to press to exit, often it's the F10 key. Once Windows starts, you will need to change your time, date, and year, but this setting should last for years before it starts becoming wrong again. If it is doing the same thing again, then either the new battery is a dud or it's not the CMOS battery after all, but something different.

It's really easy, as long as the battery does not stay out of the computer for very long, and the voltage of the new is exactlly the same as the old. If you are not comfortable doing this, take the box to a computer place and they may do it for a (hopefully) low price.

If you want, PM me if you want me to help you through it. I'll give you my home number and walk you through it if you wanted.