There are several checks listed in this thread:
http://techrepublic.com.com/5208-111...hreadID=193894
Assuming that you haven't altered the hardware and it was working previously I would suggest clearing the CMOS as a first step.
To do this unplug the computer and remover the motherboard battery. You may also see a 3 pin jumper next to the battery perhaps labeled JBAT. If so move the jumper from 1|2 to 2|3 for a few minutes. Then reset to 1|2 and replace the battery.
As another check test your Boot discs on another computer to see if they will boot.