I have tested this with a network GPO and the enviro var is not present at the moment the script runs. It is however there after logon if the same script is manually run from the desktop. I think you have no recourse but to put it into a different script run at a later time in the logon sequence. You could try putting it in the 'C:\WINNT\system32\usrlogon.cmd' script or push a link to the RunOnce reg key.
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Give a man a fish and he will be back for more. Slap him with a fish and he will go away forever.
