You can switch the default system python of the same major version, python 2.5 for example, between the arcgis and o4w by creating a registry file for each and delete/import at need. For example:
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