Setting registry value "HKEY_LOCAL_MACHINE\SOFTWARE\Altiris\Altiris Agent\Run UI
in main session only" to "1" doesn't prevent agent UI from launching per each terminal session.
This is a known issue and has been reported for fix.
A permanent fix has been provided in 7.5 SP1 HF2. (See DOC7940 )
Windows 2003 , ITMS 7.5