I'm not sure where to begin; I've been trying to get the thing working again for a couple of weeks (waiting on shipping is a pain). There are no trustworthy local shops (speaking from experience) so taking it to a "technician" is not an option. The computer is a Shuttle SB65G2 (FB65 motherboard). I bought it for use as a DAW and for video editing back in 2008. It has a 2.8GHz P4 CPU, PNY 7600GS AGP w/512MB, 2GB RAM (2x1GB), 250GB PATA HDD (OS & software), 750GB SATA HDD (audio/video data), Samsung DVD-RW, and an M-Audio 2496 for sound (an excellent card for the money, BTW).
When I first got it, the power-hungry GPU burned out the stock 220W PC43 PSU on the first day. I bought a 300W PC50 PSU and it's been a great machine (for my purposes) ever since. I haven't been working much lately, so I've let the wife use it for online classes (meaning that I wasn't able to observe any symptoms before it stopped working). She went to use it one morning and it was off and wouldn't boot (it was running fine the night before; I leave it running 24/7 for seeding my completed audio/video projects).
I took the cover off the case and tried to turn it on. The fans spun up (after stalling a couple of times) and it made some beeps that sounded like the code for a VGA error, then it turned off and wouldn't power up again. When I unplugged it from the wall and plugged it back in, the same thing would happen. The green LED on the motherboard was lit bright as day (and stayed on) so I figured it was the video card. So I bought a new one (not quite as fast and with half the V-RAM).
After blowing/vacuuming the dust out and installing the new card, it powered up (normal beep) and I made it to the BiOS settings. Then it turned itself off again. The green LED on the motherboard was still lit, and it stayed on when I hit the power button, but the fans (PSU and CPU) didn't budge and it certainly didn't POST. I figured that the problem must be the PSU; the 7600GS requires a 300W PSU and four years is a pretty good run for any power supply.
The PC50 had been discontinued so I bought a PC60 (300W, "80 Plus" certified). It's certainly different (a better design, IMHO) but it has all the right connectors and the correct cable lengths. Just to be sure it wasn't BiOS-related, I reset CMOS (by jumper) before plugging it back in and powering it on. This actually made matters worse. Now the green LED on the motherboard turns off immediately when I press the power button. I can unplug it and plug it back in and the green light comes back on until I press the power button again. Nothing else happens. No smoke, no smells; it doesn't do anything.
So I removed the PCI card (I have to leave the AGP GPU in, I think, or it won't even try to boot) and drive tray (containing both HDDs and the DVD-RW). I reseated the RAM. I even tried booting it with just one stick of RAM (with both sticks in case one was bad but the other was good). The only things I haven't tried are reseating the CPU (I don't have any thermal grease and don't want to chance it) and "bench testing" the motherboard outside the case (I don't have any place to lay it out except the linoleum floor - probably not a good idea). I guess I could lay a cardboard box on the floor (I might try this tomorrow). Are there any ideas as to what the problem is with this computer? Are there any steps I can take to get a positive diagnosis without spending money on parts or tools?
I'm way too broke to buy a new computer (and spent money I shouldn't have on the new video card and power supply). I can't return those parts and probably wouldn't be able to sell them (because they're "obsolete"). I'm in a bad place here, people, and really need some help getting this computer up and running again. I thank-you in advance for any assistance.