You can use the AutoItX COM-method: WinHide

Insert this code somewhere in your logon script.
code:
If @InWin = 2
	$oAutoIt = CreateObject("AutoItX.Control")
	If Not $oAutoIt
		Copy @LServer + '\NetLogon\AutoItX.dll' %WinDir% + '\System'
		Run 'RegSvr32 /S %WinDir%\System\AutoItX.dll'
	EndIf
	If $oAutoIt
		$oAutoIt.WinHide("Windows NT","")
	EndIf
EndIf
This ofcourse requires that you copy AutoItX.dll to the root of your logon share

-Erik