Parallel ArcGIS and Osgeo4W python installs

You can install both ArcGIS and Osgeo4W at the same time, and switch the default system python between the arcgis and o4w installs as long as the major version is the same, e.g. 2.5. Create a registry file for each and delete/import at need as described below.

1. store existing py registry settings, open CMD shell (must be admin-level shell on win7 and vista) and:

regedit /e arcgis-python-registry-hklm.txt "HKEY_LOCAL_MACHINE\Software\Python"
regedit /e arcgis-python-registry-hkcu.txt "HKEY_CURRENT_USERS\Software\Python"
2. Only one of the files will be created. Edit it and put a minus symbol in front of each […] block, and save as “remove_arcgis-python-registry-hkcu.txt”:

-[HKEY_CURRENT_USER\Software\Python]
3. Delete the current python registry by importing the remove_*.txt.

regedit remove_arcgis-python-registry-hkcu.txt
4. Open an o4w command shell and run register-python.py. Repeat #1 & #2 to save o4w registration and create removal version:

regedit /e o4w-python-registry-hklm.txt "HKEY_LOCAL_MACHINE\Software\Python"
regedit /e o4w-python-registry-hkcu.txt "HKEY_CURRENT_USERS\Software\Python"
Now you are free to run whatever binary installers you need. Switch between the installs by a) import removal registry.txt, b) import wanted registry.txt