X51 Single beep, No POST

Hi there, I recently bought a part-built alienware system that was created by dell and have put in RAM, CPU and a heatsink, connected the power supply and I receive a single beep from the machine that repeats with no video output at all. The machine stalls and I hear the single beep continuously while the fans still spin.

How can I trouble shoot this to determine if the motherboard is definitely broken?

Thanks