I have tried the following modified code . It works sometimes but unfortunately not always. The problem is, that all users / PC's habe the "printer" called "Microsoft Office Document Image Writer". I do not want to delete this "printer" but it may not be the defaultprinter. The Portname of the most USB-Printer called "DOT4_001"

Code:
        $strComputer = "."
	$objWMIService = GetObject("winmgmts:\\" + $strComputer + "\root\cimv2")
	$colItems = $objWMIService.ExecQuery("Select * from Win32_Printer",,48)

;	Sleep 30	

	? 'Local Printer List'
	$count1 = 0
	For Each $objItem in $colItems
		? 'Printer Number: '+$count1
		? 'Printer PortName: '+$objItem.PortName

		If $objItem.PortName = "LPT1:"
			SetDefaultPrinter ($ObjItem.Name)
			? 'Error: '+@ERROR
			$DefaultPtr = "Set"
		EndIf
		If $objItem.PortName = "DOT4_001"
			SetDefaultPrinter ($ObjItem.Name)
			? 'Error: '+@ERROR
			$DefaultPtr = "Set"
		EndIf
		$count1 = $count1 + 1
	Next

     ? "Drucker verbunden "