Difficult to detect hardware failure

[buksida]

Same machine as the 'missing desktop' i recently posted about and managed to fix myself.

Now we're having hardware issues. No power whatsoever today when i tried to turn the beast on. Tried a spare power supply and now it comes on but stops during the POST at the memory check. Here's what I've tried:

Disconnect all hard disks and CD drives.
Remove, replace and swap RAM modules.
Tried two power supplies.

Specs as follows:

420w PSU (toast) - testing on spare 350w
ASUS M2NPV-MX mobo
AMD X2 5600+ CPU
2048 MB DDR2 800 RAM
Nvidia 8600 GT graphics card

Any ideas appreciated before it goes back to the shop!

[MobilePCPhysician]

Test the memory in another computer, if possible. Use Goldmem. Try a different video card.

[buksida]

It was the power supply, tried another one and it booted fine, thanks for the suggestions.

[buksida]

Ever since the power supply has been replaced the machine has been really cranky, all USB ports on the board have been fried too, here are the other symptoms:

1) Very slow booting time, takes forever to get from 'login' to full desktop showing.
2) Extremely sluggish program switching and general operation.

Here's what I've tried:

1) Check task manager for anything using lots of RAM or CPU
2) Full scan with Avast, Spybot, A-Squared - clean
3) Run Checkdisk utility on startup - clean
4) Full Defrag

Could it be possible that something else on the mainboard is toast and causing this?

[Ferrit]

Sounds like when the power supply blew it partially took the board out as well.
Check the capacitors for bulging.
This is just a slight chance for visual. Most likely you wont see any evidence that its gone/going