[bleep] thats not a good sign!
Ok, the next step is complicated & you must follow what I say very carefully.
Basically, you want to remove the mobo from the case (this will eliminate any possibility that something's shorting it out + allow you to inspect the mobo)
1. Wear a static strap
2. Remove all the pci/agp cards etc. Unplug from the mobo, all cabling from case, fans (except CPU fan) & drives. Disconnect the PSU entirely from the system.
Leave the CPU+hsf & RAM in place and remove the mobo from the case.
3. Place the motherboard on a non-conductive surface. I find a pile of paper,newspaper or card works well. Make sure that the whole of the underside of the mobo is covered by it.
4. Inspect the mobo for any signs of damaged components. Look at the capacitors etc for signs of discolouration or charring. Make sure none of the tracks are broken (check front & back of board). Make sure theres no faint burning smells coming from anywhere on the board.
5. Connect up the video card, PSU, keyboard & monitor to the mobo. You may have to remove the PSU from the case to get it to reach.
If you can, remopve the system speaker from the case & attach it to the board. (Obviously it helps if you make a note of all the front panel connections before unplugging them in step 2!
6. Clear the bios again
7. With the PSU connected to mains & switched on at the back, locate the 2 pins that the soft power switch (front of case) normally connect to on the mobo. Using a flat bladed screwdriver, briefy bridge these 2 pins together. This should power the system on (ie CPU fan should spin) & hopefully you will get a display up on the monitor.
Let me know what happens.