Nothing shows on Monitor

I know this has been topic has been posted before but i need another opinion. My friend gave me his friends computer to look at. Well as far as the history to that is from what i know it was going good until it either got a virus or was hacked. When you turn it on everything starts up, all the fans hard drive, cd drive etc. I have tried swapping plus reseating the ram, cleared the cmos etc. Now the clincher if i reseat the cpu the monitor starts but as soon as its the computer is stopped its back to the way it was. So that be the cpu or the motherboard thats the problem… its really got me scratching my head