I was on my girlfriend's PC tonight just browsing the internet when suddenly the screen went black and a few seconds later the tower shut down. The power button turns the tower back on but I get no video signal and the fans seem to be running full force without slowing down. I opened it up and cleaned all the dust out but that didn't help at all. I'm thinking it is either the video card, PSU, or motherboard. The board looks to be fully powered with all lights and fans running so I doubt it's the PSU. Also when it won't boot, I can hold down the power button to shut it off, but then pushing the button again results in the power light coming on but nothing else seems powered. Holding the button at this point results in power "pulses" but not full power up or shutdown. It was about 3 A.M. when all this occurred so I haven't put a lot of effort into checking it out yet. The machine is one I built myself several years ago. It has a Chaintech nForce 4 Mobo with an Athlon XP 64 CPU. Sapphire Radeon video card, Antect Sonota II case with Antec PSU. My girlfriend says she heard a loud pop about a half hour before this occurred. She thought something fell over in the next room over, but thinking back decided that it could have come from the general area of the PC. I'm pretty sure this is no virus. I had done Avast and MBAM scans a few days before it occurred with everything coming up clean. I'm going to look at it again today to see if the heatsink fan on the CPU is spinning as well as the other fans. It's plugged to the board and the others are plugged to the PSU directly since it has a built in fan control. Unfortunately there is no onboard video on this thing but I do have an old PCI graphics card I could plug in just to see if I can get the BIOS screen. At this point I can't even get to that. I'll check and reseat everything as well. I may have a spare PSU around here I can test as well so long as it has the correct pinouts. Outside of that I'll have to find the manual for the VNF4 Ultra mobo and see if there are any error codes I can check. I did pop the CMOS battery as well and reseat it with no success. Update: Some more info: 1) plugged in the main leads from my own PSU. Everything lights up with fans running just like the original PSU does. I don't think the PSU is the issue. 2) Put in an old PCI graphics card. Still not video. 3) Cleaned and reseated all connections. No good. 4) I noticed that my DVD drive does not seem to get power when I turn the computer on. I can't eject it. When I remove it and plug it into a power adapter it works fine. The HDDs on the other hand feel like they are vibrating even with all fans disconnected. Can the Mobo controller which devices the PSU sends power to? I've been told the Mobo will not POST without a CPU but it should give an error about no CPU found. In my case the monitor only says No Signal so I'm getting nothing from the Mobo. I'm almost certain the Mobo is at fault. Any additional input?