virtualenv --no-site-packages v1cd v1\Scriptsactivate.batpython -c "import django" # - no problem here

Why does it see the Django package??? It should give me an import error, right?

Just unset PYTHONPATH environment variable. The idea of virtualenv is that you can create your own environment (fully isolated or extending the default one) so you don't have to mess with that.

As someone noticed there's already been a similar question on SO. Read it if you need a better explanation.