Sounds like your CMOS battery may on its way out. It's located to the right of PCI slots 3 and 4. Replace it and see if that fixes your problem. If that doesn't do it, your CMOS may need to be reset. You could also try the following before replacing the battery.

Here's the procedure:
  1. Unplug the computer.
  2. Remove the battery.
  3. Short the to solder points by touching both with a screwdriver or other metal object.
  4. Re-install the battery.
  5. Plug in the computer and power it on.
  6. Activate the BIOS screen during boot-up by holding down the <Del> key.