It sounds like something is shorting out your system. I would try and put only your CPU in the mobo and leave everything else off and see what it does. You should get a post beep indicating that your RAM is missing. If you don't get a POST beep it is most likely the mobo or processor. If you do get the beep keep adding one item at a time starting with the RAM thereby narrowing down the weak link.

Are there jumpers on the mobo to set the Front Side Bus (FSB) speed? If the FSB is set too high (ala overclocking) it would react as you are describing it. If I'm not mistaken the FSB should be set to 200MHz, which is then quad pumped to 800MHz.

Who manufactured the mobo? Who manufactured the power supply and how many watts is it putting out? Who manufactured your video card and what kind is it?

When you mounted the mobo did you use the stand offs that come with the case? If a part of the mobo is touching the case it can cause it to short out like your desribed, though it is odd that it appears to run without the CPU.

I'm leaning toward the mobo being bad here. If the CPU was bad the HSF should still spin, but you probably wouldn't get the system to POST or boot properly.