Shared ArcGIS and Osgeo4W python install

1. Uninstall existing arcgis python (or de-register as in parallel install).
2. Open an o4w command shell and run register-python.py
3. Install pywin32 for corresponding python (*win32-py2.5.exe at the moment)
4. Put "arcgisscripting.py" in "%pythonhome%\Lib\site-packages"

Recipe derived from Upgrading ArcGIS Python.