I'd recommend using the ADM template that I posted above, either in GPO or as a NT style policy file.

I use the following script to calculate the number for the drive letter mask:
Code:
Break ON
; Calculate drive letters for registry
$sDrives="foo"
While $sDrives
	"Enter drive letters: " GetS $sDrives
	$sDrives=UCase($sDrives)
	$iRegValue=0
	While $sDrives
		$iDriveVal=Exp(2,ASC($sDrives)-ASC("A"))
		$iRegValue=$iRegValue+$iDriveVal
		Left($sDrives,1)+": "+$iDriveVal ?
		$sDrives=SubStr($sDrives,2)
	Loop
	If $iRegValue "Total: "+$iRegValue ? ? $sDrives="foo" EndIf
Loop

Function Exp($x,$y)
	$Exp=1
	While $y>0 $Exp=$Exp*$x $y=$y-1 Loop
EndFunction

Exit 0