Is it possible that the ignition switch you purchased was the wrong switch, or is bad? Did you confirm the part number of the switch?(925-04228) I am assuming that the switch is connected to the wiring harness by a socket type connector that you just plug the switch into. Since the mower ran previously it would seem that the problem is in the switch (it could be grounding the power that would trigger the the starter motor solenoid, and blowing the fuse), or the wire from the starter motor solenoid to the Starter motor is grounded, so when the solenoid turns on it just shorts the feed to the starter motor and blows the fuse. Look carefully at the ring connectors at each end of the wire from the solenoid to the starter motor and make sure they aren't touching any metal. Another problem could be if the starter motor is somehow jammed. One thing other to check for sure, does the fuse blow when you just turn it to the run position without trying to crank the engine?