In my system there is Django 1.2.3 installed system wide:
C:\\>python -c \"import django; print django.get_version()\" 1.2.3 C:\\>django-admin.py --version
I had to point the "global python.exe" to my virtualenv in my project so I created my own activate.cmd
set THE_PATH=c:\my-envs\my-specific-env\Scripts ftype Python.File="%THE_PATH%\python.exe" %%1 %%* %THE_PATH%\activate.bat
It changes the the file type association using windows command 'ftype'.