Ask your provider if they can't provide an ethernet modem first that connects to an NIC card.  If not be sure to plug the modem directly into the primary USB port on the PC. For other USB devices, as others have said, get a self powered hub. Get at least a 6 port one, and get a good one. I like the D Link. What is your Dimension Model? (it must be an older XPS or similar?). I only mention this to ensure you are aware the later dimensions like the  8100/8200 etc., and later, all have 2 additional USB ports under the front flap door.