Very strange pc problem ???

Hello,
ey everyone.
when installing a game earlier yesterday my computer randomly cut of for no reason. Now everytime i switch it on it powers on for 5 seconds and restarts itself and does the same thing.

when turning the computer on everything lights up, fans spin (on full) and it seems okay but i get no beeps, no screen nothing it just restarts again.

I have tried to narrow it down and it appears it could be the CPU, PSU or motherboard. I took out memory, replaced it, disconnected drives, removed the GPU, i tried everything but still no solution.

Any help is much appreciated. Thanks

Update we have eliminated the mobo could this mean its my cpu/processor or could it be my psu but my freind tested a totally different psu so i dont think this is the problem.

i got a brand new mobo gigabyte model and its doing the same thing but this time it seems to stay on i will make a video on youtube later on after my holidays.

Is thier anything i can do at all to get my pc to work ?? as i live in a small town nothing pc wise is around where i live and i cant go to town because its about 2 hours travel.

Thankyou will update if i find the problem dought it!

Thankyou and have a nice day much appreciated

UPDATE SEEMS IT COULD BE THE GRAPHICS CARD I GET NO SCREEN WILE THE PC IS TURNED ON HOWEVER ONCE I REMOVED MY ATI 4870 1GB CARD THE PC SEEMED TO BE VERY QUITE I WONDER IF THIS IS MY GRAPHICS CARD I HOPE NOT I WILL TRY A CRAPY NVIDIA 8600 GT SUNDAY AND TELL EVERYONE THE NEWS