Small Engines (Lawn Mowers, etc.): Troy Bilt bronco lawn tractor, pulley bolt, hp motor

Question
I have a 2004 or 2005 Troy Bilt bronco lawn tractor with a 42" deck and 18 hp motor.  Today while mowing I came down a hill and when I hit the bottom and stopped could not go forward again.  Put it in reverse and same thing.  After messing with it a bit it did move forward very slowly, but it was smoking from under the engine and it felt as if something was slipping.  If I barely pushed the pedal it would slowly move forward but was very jerky.  If I pushed the pedal all the way it was very stiff and the smoke would really pour out from underneath it.  I checked the belt and it seems to be tight (I replaced the belt about a month ago).  I started it up and got down on the ground to see if I could see the problem and the tractor began to move forward without touching the pedal and hitting the brake did nothing to stop it.  I had to shut it off and walk away before i dropped a match in the fuel tank.  Any ideas?

Answer
Are you sure the belt is not binding, or is a pulley bearing seized, or did the belt fall off a pulley?

Are the enngine mounting bolts tight?  Are the transmission mounting bolts tight?  I the main engine pulley bolt tight?  Yes, I have had all these problems.

Eric