Pentium 4 2.5 gh
ABIT VT7
radeon Video card

This system had been stable for 4 years. Now nothing starts.
The PSU appears to be dead. The fan on the PSU does not
even come on. The only thing that comes on when I turn on the
power using the PSU switch is the LED on the mobo.
There was a smell of burning coming from the PSU after the failure.

Does this indicate that the PSU is the only problem or is it also
a mobo problem?
I do not have any spares to swap in. What is the best way to procede?
If I replace the PSU is it likely to burn out if the mobo is bad?

I doubt that the mobo is damaged,but it is a possibility .It is unlikely that a bad mobo will damage the psu.However, the only way to find out is to replace the psu.

Sparkax,
Thanks for responding. I guess I'll just have to bite the bullet on this one and see what happens with a new PSU.
Do you know of any situations where a bad mobo can cause a PSU failure?

No, I have never come across such an occurrence.

most common one a bad PSU causes a mobo to be faulty, but a mobo to cause a PSU to be faulty..haven't heard of this thing.

Hello cguan77,

Thanks for the response.
All this started happening when I accidentally hit INS instead of DEL.
That made the display have a kind of "checked" look. After a few more reboots the screen went black. Fans etc. still going. I powered off and several hours later when I powered on again nothing worked and a burning smell came from the PSU, but still the LED on the mobo came on.

Well, I replaced the PSU. Fans worked and got a POST beep, still no display. At this point I discovered that the fan on the Radeon graphics card is not working. I bought a XFX GeForce 6200 card and the machine now works. Unfortunately I now have a device conflict with the Hauppauge video capture card.
Has anybody any suggestions where I could go to discuss this issue?

try changing the settings that causes the conflict...but not so sure if it will help... but before trying to do anything back-up all your important files.. it might crash windows...

or try repair installation...of your windows