|Maybe you have a PC Time Machine? I used to have a Hot Tub that did the same! ;)|
Seriously, you need to replace the CMOS battery on the motherboard. It's usually quite simple to do, depending on your motherboard. Just take a look at it and you should see what looks like a big (2cm) watch battery. It should pop out and you will need to buy another one to replace it. I think I bought my last one from eBay and they are usually pennies.
Hope that helps.