you can use a GPO without using an MSI file. just use the login / startup script options. the advantage of the startup script is the startup script runs at System level security and the login script runs at user level. so it is posable the login script may fail if you have security options set low for users as you should. I just run a .csm/.bat file which calls the agent.exe with the options /e /c /s /g=GroupName that way it installs when the computer reboots in the event it did not install any other way.