Disconnect everything not necessary to boot the BIOS (mouse, hard drive, CD/DVD drive, any cards plugged into the motherboard (other than the video card), all the RAM (memory), all external slots empty - you just want to have the mobo, PSU, CPU and cooler, video card and keyboard in. Make sure to disconnect both the power and data cables to the hard drive/CD/DVD drives.
Try to boot, if successful you'll be able to get into the BIOS (and nothing else). If it doesn't start up (or just beeps at you), put one stick of RAM in the first slot and try again.
If the system still doesn't start, then it's one of the parts that's left in the case that's causing the problem. You'll then have to troubleshoot them (not real easy).
If it does start, then it's one of the parts that you disconnected that's the cause. So, start putting things back one at a time and try it again. When you can't get into the BIOS, you've found the culprit.