I have a routine i go thru with dead puters. I first check the power supply to see if all voltages are within the range of working. The next thing i check is the ram. I keep known good sticks of various types to check with. If the puter still does not boot to the bios i now start with the basics. Sometimes i will pull the motherboard out and do a setup on a cardboard box. I have the MB, processor(with fan hooked up), ram, video, monitor, keyboard, and mouse hooked up( nothing else). If i do not get to the bios then i try a different video card. On some MB's i have to have a floppy drive hooked up or i get POST beeps and no bios(older MB's). At this point i have to decide wheather to continue. Puters that are less than a 1 gig processor i usually strip them down and save the good hardware. Most of the intel puters that used RDRAM( usually the 1.4 to 2.4 processors) i also junk and use for parts. If i have the spare parts to continue i will try them. If i don't have the parts i cannot afford the time to find them and usually junk the puter. New puters are so cheap now it is not worth spending a lot of time trying to fix them. i usually build a new puter every90 days. It seems lately some people have found out about me so it seems i am having a shorter between time before building another puter. People who come to me do not want a regular computer. So i build them what they want. My basic computer starts at $800 and goes from there depending on what hardware they want.
All i'm really trying to say is to not spend too much money on a old puter. It doesnt take much to have the cost of a new puter tied up in a old one. Parts for older puters are getting harder to find and the people who have those parts want higher prices for them.