HP Vectra XU 5/90 dead ?

Recommended by Wim's BIOS...

At power on, the three led on front panel light orange and nothing append (no sound exept the starting HD).

I have got change CPU, memory, video, disk and nothing more append.

I have got verify +5V and +12V (OK) but can't change powersupply because it's not standard ATX connector size (2 more pins / total of 22)

I think something is dead, motherboard or powersupply but don't know wich of them.
The powersupply is almost ATX but the power switch is a toogle and not a push button.
Sometime, you can't switch off, the power switch do not have any effect.

Any idee?