Before you take the MB out and all that. The fact that the Power Supply fan is not running is a critical point.
Modern Power Supplies are started through a contact trigger and this is probably where the problem lies.
The ON/OFF switch is really just a trip switch to send a signal to the PS to go active. Providing there is power at the back and the little switchat the back is on (excuse me being pedandic - but I have lost precious hair over this myself). Check the connection to the bottom of the motherboard to your Power Switch - and the main power connections to the board. Remember that P4 boards now use the long block connector (pretty hard to get wrong) and a small +5v/-5v 0v square connector. The power ON/OFF connector one is not polarity sensitive, but those little pins can be sods to identify.
If you are convinced that all the above is OK - then the next obvious suspect is the Power Supply itself. Borrow, beg, rob another one and try that.
Then bin the MB....