HP Slimline s5360f problem

My HP Pavilion Slimline s5360f was working fine, then the fans started intermittently started spinning like it was going to take off and fly. I read to open it up and clean the dust out, so I did. Plugged it back in and no picture as if cable was disconnected. I checked all the connections inside and out, took it to another monitor and still nothing. So, I bought and installed a new video card GeForce 8400 GS and saw that I had to have a higher power supply, so also installed a new 440 PS. Hooked it all up, and still no pic as if cable disconntected.

The computer powers up, fans come on and it sounds normal, light comes on on the power button like normal. Interestingly, when it was working, you could push and hold the power button to turn it off. It won't respond to that now, only turn off option is unplug it.

I'm way out of my league here... normally work on people, not computers. Much easier to do surgery on people that computers.

Thanks in advance,

Dr C

Of course, just out of warranty. I have it backed up with carbonite, so data is safe. Is this a trash it and buy a new one? Or new HD? Not sure what to do.