I have a Windows 2008 domain with Vista workstations.
I don't want users to have admin rights on their machines, but I want to allow them to install a specific company application.
This application is signed.
I would suggest that group policy would be perfect for this - the software is installed at logon (usually invisibly) and the users need no additional rights at all.
This link at Microsoft gives an overview.
Create an AD group, such as "Corporate Application". Add the machines that you want to receive the app to this group.
Create a Startup Script which installs this app when the computer restarts, add the startup script to group policy.
I haven't used them but you might be able to do this with Software restriction policies
I've deployed non-MSI installers using the Symantec deployment tool included with the anti-virus corp edition. When selecting a package to deploy just pointed it at the setup.exe file that I wanted to have installed, instead of the anti-virus client and ran though the same steps as if it was the symantec anti-virus client that was being installed.
Don't know if you have symantec corp but if you do, thats an option.