Reset your CMOS using your CMOS reset jumper located on your motherboard. If you are not familiar with what it is or where it is at, you should be able to find the information in your motherboard manual (if you still have it). If not, the manufacturer's website probably has an online manual.
If you have a store-bought PC, you can probably call tech support and ask them where it is or use their website as well.
Aside from clearing the CMOS, you may need to reseat or replace your system memory modules... a DIMM in your motherboard might even be bad... But usually resetting the CMOS fixes this problem.