@iki: could you refactor your code so it's in shutil (and the tests in test_shutil)

few remarks: only which/which_files should be public API,

Next, I don't think extensions like VBS should be hardcoded if PATHEXT is not found. A pseudo-dos shell just runs .exe and .com IIRC, if not, there's probably somewhere in the win32 environment a list of extensions and their associated programs that get called automatically from the shell (in addition to PATHEXT). We need to find this list programatically rather that harcoding a list that will change from one box to the other.