I suggest a good starting place is
Oshon Software Homepage, download the trial "PIC Simulator IDE" and load the hex file into the simulator. It will not only disassemble the code, it will let you run it one instruction at a time in a virtual IC. You need to register the program (not expensive) for full functionality but just to extract a listing I think you can use it in demo mode.
You can also do it in MPLAB but the procedure is more complicated and the program is many times bigger.
Brian.