Do what Tony says.

If that's not a goer, you could pull the RAM too. You have 768MB so there's at least 2 sticks in there. Pull it all out. If it then powers on with something beeping (probably long beeps), try the RAM one at a time.

Is the monitor saying cable disconnected or no signal? My monitor seems to be able to tell the difference between "plugged in and PC off" and unplugged.