That is quite a change.
Could I suggest some changes?
Maybe an indication to the user when the listbox is populated.
Also silence the screenoutput for GetDefaultPrinter().
Code:

$Form.Center 
$Form.Show 

$RC = GetDefaultPrinter() 
EnumPrinters() 

$Instructions.Text = "Your printers are listed. Now you can add extra printers and change the default."

While $Form.Visible 
  $nul = Execute($Form.DoEvents) 
Loop 
Exit 1

The ListBox does not get populated because $Listbox2 is being referenced.
Code:

	$GetDefaultPrinter = Join(Split(ReadValue("HKEY_USERS\"+@sid+"\Software\Microsoft\Windows NT\CurrentVersion\Windows","Device"),',',1),'') 
	$Listbox2.additem("Default Printer = "+$GetDefaultPrinter) 
	;Create blank space between default printer listing and the listing of all printers. 
	$Listbox2.additem("")
...
		$Listbox2.additem("Printer = "+$printer)