I noticed the Adobe Extension Manager that ships with Creative Suite was giving me an error when I would try to install an extension. It turns out you have to run it as an administrator. I would assume this is the case on Windows Vista as well. Here’s how to make it (and any other program for that matter) permanently run as an administrator.
Right-click on the program in your start menu and choose Properties:
Under the Compatibility tab, click Run this program as an administrator and click the Change setting for all users button:
From this point forward, you always get that annoying User Account Control dialog, but at least your extensions will always install.