Difficult to detect hardware failure

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!

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

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

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?

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