I can reproduce the problem under Windows 7.  Thank you for your example and description -- they were very helpful.

Detection that the original parent was PythonService.exe is necessary to avoid undoing the paths set appropriately for running under a service.  The current code only detects the immediate parent.

Modifying get_preparation_data() in lib/multiprocessing/forking.py to perform an additional test on the inherited values from the original parent's sys.argv (available via the preparation_data key 'sys_argv') would resolve this:
    if not d['sys_argv'][0].lower().endswith("pythonservice.exe"):
        ....

Potential complications to existing code appear very unlikely given its nature.

Patch forthcoming after running tests unless someone wants to beat me to it.