Is there a way to silently or automatically install .NET?
Use the "Chaining the Redistributable Package in Application Setup" description in the article below. The /q switch should allow installation of the framework without user prompting. This should solve the problem of your users cancelling the installation of the framework.
.NET Framework 3.5 Deployment Guide for Application Developers
http://msdn.microsoft.com/en-us/library/cc160716.aspx
Of course, here's a blog post describing the command lines. Obviously if you're deploying your application with an installer, you'll want to use the recommended approach Robert describes. But if you're just trying to install it on a bunch of machines this may help.
http://blogs.msdn.com/astebner/archive/2009/04/16/9553804.aspx