Pavel314 wrote:
I have a two year old Sears Craftsman lawn tractor with the 42" mowing deck.
It works well with one annoying exception. Almost every time I want to mow,
the drive belt has slipped off of the power take off pulley and is hanging
slightly below it.

I had the same model for 5 years now and the belt never slipped off
unless it was caught in a twig. Something is bent or out of alignment,
IMO. If you have a way of lifting the mower it should be easy to see
what's wrong. Take precautions, that thing is too heavy to be careless