Since your computer has to keep its clock running even when the PC is turned off, there's a battery inside it. If the battery starts to fail, you'll find that the clock doesn't work properly. It's important that you change it, as your computer will start showing errors if, for example, it thinks that you're trying to revise a document before you actually created it.
Fortunately, it's usually a fairly simple (and inexpensive) job:
http://www.computerho...m/issues/ch000239.htm
Chris