I had a system up and running that died one fine afternoon.. Very sad.
I press power, it starts and the fans begin to whirr--like any other boot--, and then it all shuts down after 3 seconds or so... It never POSTs.

She was: (barebones listed)
Biostar M7VIT Pro motherboard
Athlon Thunderbird 2100+ CPU with Thermaltake Heatsink
2x 512MB PC2100 DDR RAM, Crucial Micron and Kingston
Radeon 9500 PRO AGP 8x card
ATX Power Supply

First, I thought maybe the power supply was dead -- so I went to Breast Buy and got a new one. No good.

Then, I mail ordered a NEW motherboard... ASrock K7S41. Installed everything -- but same problem. Begins to boot for 3 seconds then shuts down without POSTing.

I have troubleshooted the RAM so far by only using 1 of my 2 sticks -- tried each separately, but did not avail.

I have tried booting without the AGP gfx card installed -- same problem. But, I don't THINK my mobo has on-board gfx so this may not be certain troubleshooting.

Is there anything I am overlooking? How can I troubleshoot my CPU or AGP Gfx card?

Thank you.

Only way to troubleshoot CPU is to replace it with new or verified good one.

Only thing I can think of for troubleshooting here is IDE/SATA drives. Disconnect them all (I mean the data cables). Dead optical drive can cause this behaviour.