I would try another power supply, But assuming you don't have an extra one. Try Posting with only the Minimum, CPU, RAM and Videocard, Then if that works. attach the HD and Boot, Continue to attach devices until it no longer works.
Now obviously you NEED to turn off your computer before attaching the next device.
If it doesn't even Post, with CPU, RAM, Video card then were looking at PSU, CPU, Motherboard or a RAM Problem.