damn~~ u had exactly same problem as i did!!!
First of all listen carefully when u boot up the computer and see if u hear hard drive is running, if not, its 99% either mainboards or VGA problems.. However, try someone else's VGA card and see if it works with other VGA's. In my case, i tried both of them and it didnot work, so i call intel(my motherboard was made by intel.) and they told me to do something about took off motherboard battery and put it back after 20M, unfortunately, this one did not workd neither. So i just purchased the new one and it worked...
I know my case is extremely rare but this thing could happen to you.
So my advice is change ur video card and see if it works. If not, i bet its one of those motherboard problems.
Good luck!