This sounds like a power supply problem then if the monitor is functioning okay. Your local computer shop should be able to sell you one for $20-$35 depending on your systems power requirements. If your up to it, remove your power supply and take it with you to the store so they can sell you an equivalent one.
Another culprit might be the motherboard and an overheated chip. This is far trickier to troubleshoot and unless you’re comfortable taking out the entire innards of your computer, take it to a store that does computer repairs.
I’m not an expert, but I’ve seen this sort of thing before. If the system’s starting up, even in that limited way, is it likely to be a power supply problem? It sounds more likely that something’s stopping it booting. I would think it could be an overheated chip, but it could be something even simpler.
One thing you could try, which is quick and costs no money, is to open up the computer and press all the major chips (especially the CPU) and PCI/AGP/ISA cards, etc, firmly into their sockets. This often seems to do the trick.
It might help if you posted a little more detail about the ‘no video’ start up. Does it seem to go through the normal start-up, just without video (typically there will be beep, you’ll see the keyboard Caps Lock and other lights flash, hear the drives spin, the floppy will check to see if there’s a boot disk in it, etc.) ?
Or does it just stop booting up before all of those things happen?
That would be my first guess too, except for the 40 minute thing. If it were a peripheral card not all the way in, it shouldn’t boot at all, not just after 40 minutes. It sounds like something is cooling down in that period, so I guessed the cheapest parts to try first.
effac3d, when the computer does finally start up, does it start normally, i.e. the hard drive doesn’t grind for a long time, monitor comes on okay with video signal, etc?
Meh, I’m still thinking in corporate IT mode rather than as a home user.
You can try these troubleshooting steps to see if you can isolate any hardware problems:
Make a bootable floppy.
Remove all peripheral cards except the video card.
Disconnect all IDE devices (hard drive(s) and CD drive(s)).
Remove all cables from the back of the computer except power, keyboard, and mouse.
Power on the computer with the bootable floppy inserted.
If you get to a DOS prompt then one of the things you disconnected/removed is the problem. Start putting things back on one at a time and power on the computer (please be sure to disconnect the power cable when reinstalling internal devices). If it fails to boot up then your last item connected is the culprit.
If you don’t get a DOS prompt when everything is disconnected, then the video card (but you’ve already swapped that out), motherboard, CPU, RAM, power supply or a heatsink/fan is probably the culprit (I’ve never seen a keyboard, mouse, or floppy drive overheat so those are low suspects, but ya never know). Unless you’ve got some of these hardware items as spare parts laying around the house, your best bet is to put it all back together and take it to your local store to let them test it out.