GPO software deployment without WMI?

Man, I can hardly imagine how many things in your environment must be broken if remote WMI can't be used, even from domain controllers, SCCM servers, etc. Sounds painful. But anyway,

According to this article:

http://technet.microsoft.com/en-us/library/cc782152(v=WS.10).aspx

No it doesn't rely on WMI. As long as the Group Policy client can access Sysvol, and access the software distribution point (which may or may not also be in Sysvol,) and the Windows Installer is in good working order on the client, that'll do. Also note that software deployment doesn't work over a slow link - so make sure a fast link was detected.

Edit: Also check this question for some other basic tips for troubleshooting software deployment failures:

Deploying .msi application through GPO doesn't work

Oh, and finally... try turning off your AV and see if it works then. Then you'll know if your AV is breaking it or not.