I think this should cover it all
I removed the Error parsing NTDOC suggested earlier because I want to keep the function as simple as can be. It returns an error code, ppl who use this function should be able to recover the error themselves.

Code:

Function ChangeVLKey($VOL_PROD_KEY)
  Dim $obj, $objects, $x
  ;Key is without hyphens (ABCDEFGHIJKLMNOPQRSTUVWXY) 
  $objects = GetObject("winmgmts:{impersonationLevel=impersonate}").InstancesOf("win32_WindowsProductActivation")
  If @Error <> 0
    Exit @Error
  EndIf
  For Each $obj in $objects
    $x = $obj.SetProductKey($VOL_PROD_KEY)
    $ChangeVLKey = @error
  Next
EndFunction