Cant boot!!!!!

i cannot boot my computer, i have disconeccted everything but de video card, memory. So no hard disk or cd, when i turn ON the computer the light turns on and the fan and i think i can hear the processor running, but my monitor doesnt shows a thing, actually suddenly, by making no change i the Dell screen appeared and i could enter the cmos-bios, and everything was fine, but it only happened for about 4 times, and then back again to the initial problem, ive also taken out the cmos battery for around 6 minutes, and then back again, i dont know what can it be, PLEASE HELP!!!