not booting up?

Sounds like it could be the motherboard at fault but first unplug all devices from the ports including screen keyboard and mouse just have the power lead in nothing else does it work then if so connect devices one by one untill you find the faulty.
otherwise disconnect all drives card readers and expansion cards and try again if still no sucess then remove the memory and try again
after that the only other thing you can check easily is the switch trace the wire back from the switch to the motherboard. check the connector on the board is labeled as power switch unplug the switch from the board and join the 2 pins on the board together i normally use the tip of a screwdriver to just touch the pins to make the pc turn on.

if none of these point to another component being at fault then its most likely the board or cpu assuming of cpurse that the new psu isnt duff like the last one

A
February 2011