There are some really good reads on the net about repairing switching mode power supplies. You can fix your old one as long as the transformer is good. Start by testing the resistance on the outputs to see what happened. It could have bad capacitors, rectifier, switching transistors, or all of the above.
To be honest I wouldn't bother. Just grab a board out of a working AT power supply and swap it. Be careful of line voltages when you are working.
-Jeff