i just had this problem a few minutes ago trying to test out a new computer. i followed microbell's suggestion about putting mobo on cardboard and tested the system outside the case.
actually i had no case or power supply for the new computer and was too lazy to swap out the mobo. i turned it on and all i got was fan spinning, no video. was a bit worried so i installed the computer into another case.
the only thing i changed was took out the DVI-to-VGA adapter on the dual-display video card which was connected to nothing. the thing booted okay. i learned nothing, and my adivice is
connect only motherboard, processor, memory, and video. you do not need a hard disk. it will still boot to a error on POST. you do have to at least connect the power switch pins or you can use a screwdriver to complete the circuit to power you machine on.