Contact your motherboard manufacturer to see if upgrading your bios is necessary. In many instances, upgrading the bios is necessary in order to enable support for new products.
Verify that the motherboard is installed correctly and is not shorting out on the underside of the chassis.
Verify the memory modules are the correct match for your motherboard and that they are seated properly.
Verify that all add-in cards are seated properly.
If your PC uses a Boxed Intel® Desktop Processor in the LGA775 package, be sure the heatsink is properly installed and 'locked'. Pay particular attention to the proper orientation of the locking pins and give a good push down on the pins (sounds like a double click) to lock them into place. Rather than locking the fasteners in a circular fashion, it may be helpful to alternate locking the fasteners.
If the Intel® boxed processor heatsink has been previously installed, make sure all fastener slots have been reset to factory default by aligning each fastener notch to be perpendicular to the heatsink.
Verify the chassis/case and power supply are appropriate for the processor model and the motherboard you are using.
Verify that the thermal solution is adequate for the processor and frequency of the processor.
Make sure the processor fan cable is connected to the correct fan header (specifically for the processor). Refer to your motherboard documentation for more information.
Make sure that the thermal interface material or the thermal grease is applied to the processor properly.