Is there a way, in Windows, to prevent the user from installing all of the 'free' security scan or anti-virus software that seems to come bundled with so much free software these days? We provide anti-virus software for our users, but they often install software which adds another AV program, which slows the system down pretty badly.
Personally, I would be not letting them install software at all, just so you know exactly what's going on your systems.
There's a lot worse out there that could find it's way in with an install!
If it's "fake" AV software, using spybot's immunize feature and SpywareBlaster will prevent some cases of installation. Both are essentially killbit solutions.
Another solution could be not allowing users to install anything--give them a power user (or lesser account) and lock things down via group policy.