If you open up the side of the case you should see a silver disc shaped object normally towards the bottom corner of the board. This is the CMOS battery which probably needs replacing. Remove carefully with the power off and disconnected. Replace with the correct spare the right way up. These are available from most electrical outlets. Look for Lithium batteries. When you restart you will need to set the time and date and some of the other settings unless you accept the defaults. This solves most problems. If you still have an issue then the motherboard is suspect. If you need more information then read the number from the board and repost the answer.
John
December 2009
There is a very good chance your CMOS battery has died, but lucky for you, it is very cheap and very easy to replace.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix desktop pcs.