I am writing this on a machine that I call Hagar (from the old days, when I named computers after biblical harlots and concubines, which in turn lead to the term "calcubine"). A few months ago, this machine decided to stop booting. It's been having trouble for a while, and so I figured that it was suffering a progressive sort of failure, and was about ready to write it off. The problem is that she's a good machine, second only to my main system (and a far second at that), and I didn't want to let her go.
So, today I decided to fix her. The immediate culprit was the power supply. She came with a 90 W ATX power supply, and yet at her peak, she was running two 7200 RPM hard drives (80 GB and 20GB), a 32X10x40 CD burner, a DVD ROM drive, an ATI Radeon 7000 PCI video card, a Sound Blaster Live! card, a PCI NIC, a PCI IDE controller, and 192 MB RAM. All told, that probably pushed it far beyond the meager capabilities of the power supply, and so while it had been sans 20 GB hard drive and IDE controller for some time, and without the DVD ROM for a few months, the poor thing was probably cooked.
First, I pulled out or otherwise removed power from everything I could. The thing still wouldn't POST. I then turned to a spare PC that I had lying around, and was delighted to find that it had an ATX power supply (the system in question is a Pentium 200 MHz machine). So, I had my first experience installing a power supply. The new 250 W supply soon had the machine POSTing, but it wouldn't boot.
At this point, the only error I was seeing was a memory size error. The machine was finding 184 MB of RAM, instead of the 192 that should have been there. The missing 8 MB were being used by the machine's onboard video card (death to onboard video), and the machine wasn't even making POST with the ATI card in, so I stole the modest card from the machine that was already short its power supply, and was trying to get it to boot with that. No luck.
Company came over, and so I was away from the machine for about 6 or 7 hours, at which time it still hadn't booted (no surprise). However, after a few resets, I was amazed to see the POST finish up and Windows start up. Unfortunately, the video card didn't install right and was having resource conflicts with a phantom device. It refused to work and disabled itself, citing conflicts that it staunchly denied on its resources tab. The troubleshooter was quite confident that a reboot would fix the problem, but two reboots failed to. I suspect that it was conflicting with the Radeon card that it was hoping would still be there.
So, in a spirit of exploration, I popped the ATI card back in, and was soon greeted with a beautiful screen at a numbing 1280x1024 resolution (and on a 15" monitor, no less!). The machine is running again, and will hopefully be ready to serve as my main server box once I have my own personal network again.
Side note: This machine was originally manufactured by a well-known company that makes "affordable" computers (although the only remaining original parts are the case, motherboard, processor, and a third of the RAM), and like every PC that they sell, it came with a small sticker that served to discourage people from opening it up and playing around with the parts inside. Now, after some tribulation, I have come to realize that this was a wise warning, for while I may have been competent to install more components inside this machine, it was certainly not equipped to handle those components. The moral: Don't buy computers with cheap power supplies.