New Power Supply, still won't boot

[mofo]

My dad has a Dell E521 pc, was having what seemed like power supply issues, he pressed the power button but pc wouldn't boot.

I swapped the power supply for a brand new one (now 400w instead of 305w) but the problem persists.

On the mobo there is a steady amber led for "aux power led".

The green power led on the exterior of the casing lits up ok.

The green light shows on the usb keyboard also.

I have replaced the mobo's cmos battery just in case because it's old.

I can hear the hard drive scratching/working, and the PS fan is working.

But I don't get any audio beep from post, and the monitor doesn't turn on either.

The PC is running Vista, and using the onboard graphics, no cards.

Any ideas what/where to look next?

tia.

[glc]

https://support.dell.com/support/edo...N/adtshoot.htm

Read the "Diagnostic Lights" section.