As Phil just pointed out, you need to reset your bios. The easiest (and least technical!) way is to remove the power cable and the silver battery (about the size of a 2p piece) and leave it out for about ten seconds before replacing the battery then the power lead. Alternatively you should look in your motherboard manual for CMOS CLEAR. This will involve moving a jumper (a birghtly coloured piece of plastic) to cover two pins on the motherboard which is known as closed position and then moving it back to an open position.
This is also fairly simple although can be a little fiddly and once you've done it, its very easy to do.
Ben, March 2008