change windows system path from cygwin python?

[gry at ll.mit.edu]

[Windows XP Pro, cygwin python 2.4, *nix hacker, windows newbie]

I want to write some kind of install script for my python app that
will add c:\cygwin\usr\bin to the system path.  I don't want
to walk around to 50 PC's and twiddle through the GUI to:

My Computer --> Control Panel --> System --> Advanced --> Environment


How can a python, or even a .bat script modify the system PATH?
It doesn't appear to be in the registry.