Did you check the Keyboard and Mouse the same way?

Its likely one of three things:
A: The power supply is shutting down because of improper regulation.
or
B:the motherboard is shutting of the power supply because it detects
an error in the power.

These will appear a bit differently depending on wether its an AT or
ATX pwr supply and case.

or
C: The mother board is fried at worst. But I would bet not. Maybe a
blown Memory Card but that would normally give you a Beep, so not likely.