Deploying the v9 agent through Group Policy is easy, v10 & v11d isn't.

Deploying the Backup Exec agent through Group Policy requires that you have an MSI file that can install unattended.

Version 9 of Backup Exec was able to do this easily.  The RANT32 directory included the file:

VERITAS Backup Exec Remote Agent for Windows Servers.msi

The MSI requires a language code transform file for the installation to be successful:
1031.MST: German
1033.MST: English
1034.MST: Spanish
1036.MST: French
1040.MST: Italian
1041.MST: Japanese
1042.MST: Korean
2052.MST: Chinese (Simplified)

See also: http://www.appdeploy.com/packages/detail.asp?id=573

Versions 10.x and 11d of Backup Exec changed how the agent installs.  There is currently no known way to deploy these versions through Group Policy. (If you have a method, please let us know!)