It sounds like the power supply might be either faulty, or being burdened by a fault causing an unwarranted load. Almost always, such a fault would cause the PSU to shut down...maybe this is somehow on the cusp of doing so, and oscillating as a result. Getting rid of everything out of the equation used to be the way forward. Bare board, apart from CPU and memory, and then see if you got the POST check beeps. Nowadays, some of the bits are very expensive and it's a toss-up between not interfering with them or risking them being damaged by spikey power. Tough call.
Do the keyboard light flicker during the attempt to start?
Some time ago I posted on a job I did on my Son's workstation. It was a twin Pentium Pro, with a graphics card that cost more than a car. It was total no-go. The point of this is that it turned out to be the most simple and unlikely culprit. I'd stripped everything out - including the CPUs - before the fault showed itself. I'll try to find the link.
Motto, lots of thinking, and minimal stripping. Though I'd never have guessed at this one.
Did the computer give any sounds on the internal speaker on start up - prior to the fault? If so, does it still do it?
USB devices have a 'hard' 5v supply to them, so try removing any USB device, then any PSII type things like older mouse or even keyboard. They are easy, so it's worth getting them out of the equation.
It sounds daft, but also try disconnecting the speaker if it's an off-board type. You'll see why if I can find the link.
Do you feel confident about removing devices from the board? Sadly, so much stuff is on the MB these days that there is often not much to pull. But disconnecting the power to CD etc., is one step to take before the more serious stuff.
Memory might be reduced to one strip. No go, try the other strip.
If you have to tackle the graphics card, you'll have to look for other clues to whether it's working or not. It would be nice to leave that in till last, so that you can see what's going on, but you should be able to tell if the system comes back to normal just by the sounds of speaker/hard-disk/other fans etc. The hard-drive warning light flicker should be a rough indicator of normal booting.
That's enough for the moment, let us know how you get on.
Here it is, different, but shows the pitfalls. The bit about nuts and bolts in the works, is as true today, as it was in Marconi's time.
http://www.pprune.org/computer-inter...ml#post1805980