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OK, so first you have switched off modem and PC. Then you've disconnected the USB leads (from modem and PC), and plugged in the ethernet (both modem and PC). You have then switched on the modem, allowed that to settle, then switched on the PC?
What OS are you running. Check the drivers for your network card in the PC are current and enabled. (look in device settings). Try the speed setting of the network device in the computer too, sometimes they don't autosense properly the 10mbit speed of the modem and try to use 100mbit.
Firewalls often cause problems, try disabling them and see if that makes a difference.