If you're confident working on electronics and mains voltages, these power supplies are easy enough to repair. These switchmode supplies all operate on the same principle of rectifying the mains to a DC voltage then pulse width modulation switching it through a high frequency transformer. Often different makes/models have common issues of input bridge rectifier and/or switching transistor failure, which usually fail to a short circuit state. Equivalent parts are usually cheap and easy to source. I've written various notes on generic power supply repair on
this page under 'A3000 power supply common faults'.
Those notes are written around 230VAC supplies. For 110VAC, reservior capacitor DC voltages should be around (110V/sin(45))-1.2 = 154V.
When you say the supply is dead, I gather the fan is not running and there's no 5V output to the main board?
As for the keyswitch, it won't prevent booting if it's in the locked state. It only disables keyboard input and mouse movement (not mouse buttons). If unsure, disconnect the keyswitch connector at the main board which leaves the machine in an 'unlocked' state.
