It sounds like Comcast gave you a broken second hand modem. I'd speak to their helpdesk about it. It might just be their poorly made software that needs the USB. Either way they should never have given it to you.
This modem is almost $50.00 and it's newer and faster than the modem you have i would recommend upgrading to it. I took the liberty of finding it for you it is a nice modem and i think you should just give it a try. http://www.newegg.com/Product/Produ...N82E16825122106 It has also had awesome reviews and not many cons, so take a look and see if you like it.