Just add the path to the folder containing any package you wish to use into your PYTHONPATH environment variable (create a new one if there isn't one already) and voilá!
For instance, suppose you have a package named "foo" in a directory "/path/to/project/src/foo". Simply append "/path/to/project/src" (note that I left "foo" out of the path because that's our package) to PYTHONPATH and then open an interpreter and try "import foo". This should work both on Windows and Linux too ;)