When using active directory to assign MSI's the following applies:
1. When the MSI assigned to a computer and not a specific user, the installation of the software occurs the next time any user boots the machine. However, the software can be uninstalled only by a user that has administrative privileges on that machine.
2. When the MSI assigned to a specific user and not the machine, the installation of the software occurs when the user launches the shortcut from the Programs Menu or installs via Add/Remove programs in the Control Panel. Now, the can uninstall the software (user without Admin Privileges), but it will remain advertised unless it is unassigned from Active Directory.
Hope this helps.