Symptoms are what is described in the title. The machine reacts to the power button being hit by spinning up the two fans: CPU and PSU. The hard drive (SATA) spins up as well. No other reaction.
This one symptom is particularly weird, though: the optical drive will not open with the IDE cable attached, but if I unplug it from the mobo it will. I can turn the PC on with it attached, won't open; then unplug IDE while it is still on, WILL open; then plug IDE back in with the PC STILL ON, WON'T open.
I have disconnected every peripheral unnecessary to POST. These include: mouse/keyboard, PCI modem, the IDE optical drive (power and data), and the SATA HDD (power and data). Video is onboard. The only two things connected are DB15 video and power cable. There were 2 512 MB DDR2 sticks of RAM in it. I have tried running it with just one of them, then switched the other in. Currently seated is a completely different 1 GB stick that I keep around for troubleshooting purposes, and I have tried it in both slots. I have replaced the CMOS battery with a used one I had lying around, and which worked in the computer it came out of. I have tested the PSU with a tester to confirm it was good, then tried connecting another PSU just in case--same symptoms. I have even tried a suggestion I found elsewhere on this site wherein one disconnects power from the PSU and then presses the PC's power button twice, thereby "resetting" the PSU.
Currently I am trying yet another suggestion: turn it on and wait an inordinate amount of time for POST.
Any help would be appreciated.