You shouldn't need any ntl install software if you are using ethernet. you only need that if you are using USB, as the USB drivers are not standard windows.
When you change connection from the USB to ethernet, or vice versa, you must switch off the power to the modem, leave off for say 30 seconds, then power on the modem, let that settle and only then boot up the PC. Only the ethernet, or, the USB cable can be plugged into the modem, not both.
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This article might help you:
http://www.cableforum.co.uk/board/article.php?a=13