Preventing users from installing Apps

We are rolling out 100+ XP Pro machines. I am creating a Ghost image of a test machine we will use on all the others. We want to restrict the ordinary user from being able to install anything on their computer. The problem is: A certain application ( an email client called ExpressIT ) will not run unless the user has Power User, as an example, rights. I've learned that there are Computer application installs ( such as Office 2003 ) that are available to all users and User application installs ( such as the one above ) that require rights equivalent to the user that installed the app for it to be run by everyone. I've edited the registry to allow the ordinary user account permissions to the applications' directory, but this didn't work. I've configured the computer policy to only "Allow installations with elevated privledges" but this didn't work. Any ideas??