Most likely the battery on your system board is kaput. They are cheap, easy to find at most electronics retailers, and pretty simple to replace.

After opening the case of your system unit, look for something about the size and shape of a quarter mounted on the system board. Usually the battery sits under a tight clip and you can remove it by lifting the clip. Sometimes they snap into a socket and you have to push a little tab to release them.

After replacing the battery, start up your computer and press the appropriate key to enter the BIOS setup screen. (Usually it's the DELETE or INSERT key. Watch the screen for instructions or consult your manual.)

Set the system clock to the correct time, then save and exit. If your computer works normally now, you're all done. If it doesn't recognize one or more hard drives, go back into Setup and look for the hard drive "Auto config" or similar. Sorry if that's a little vague, but every BIOS is different, so it's impossible to give more detailed instructions. Poke around a bit, you'll find it!