I suggest that you should try to boot the PC with minimum configuration ie. disconnect everything except the Video Card, RAM and one boot disk and also double check:
- Your Video card 260GTX requires 2 6-pin power cables connected. If not, you'll get no video.
- reseat the RAM. If RAM is loosely seated or bad, you'll also get no video.
- Physically inspect the CPU heat sink to make sure it is tightly connected to the CPU. If not tightly connected, the PC will shutdown immediately after a few seconds.
- If your MB came with dual BIOS then there should be a switch on the MB. Try using the other BIOS.
If still cannot boot. To make sure there's no short in the case, You might have take out the MB from the case and try again.