There is a decompiler for Windows. Its called IDA. It costs a skrillion Deutsche Marks. It will disassemble a PIC executable (but not of course decompile - to do that it needs to know about the compiler).

I believe the Oshonsoft line of simulators will disassemble - they have a trial version.

If you Google for "PIC disassembler" you will find myriad on and off line PIC disassemblers but not decompilers, and if you did find one it would be to C.

All things considered unless you only need to make tiny changes to the device in question, better of understand its function (as a black box) and write that up in Basic.

George