See if there's a BIOS update for the motherboard. The LGA775 socket covers a huge range of CPUs, form the Pentium 4 to the Core 2 Quad. Earlier motherboards may not know how to communicate with a newer CPU without a firmware update.
It can also turn out that the motherboard was abandoned by the manufacturer for a newer model in response to new CPUs coming out. In which case you're out of luck.
Edit: Ninja'ed by 55 seconds.