ATX supply, you ground the green wire from pin 14 to start it. AT supplies are either on with the switch or (IBM Aptiva and similar) you tie the black and white wire together on the 3 wire connector. A paper clip in the connector holes works.
Dell ATX supplies have a different pin out but the green wire is still "start".
These things have internal overload protection that shuts them down so it is hard to blow one up. Some will shut down if the load changes suddenly.
