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Firstly if your tried running a few things, and changing settings around we need to ensure that everything is set back to default and work from there.
On the router, follow the manual instructions and do a reset to factory settings.
On your computer, check that the Internet connection post has the TCP/IP properties of the port set to auto detect so that the IP can dynamically be allocated. In network connections if you have setup any dial up, or other connections remove them. The only ones you should have in there will be your network card, and maybe a firewire port. In Internet Explorer, again make sure that the browser connection in options is set to auto and dial a connection is unticked, with any other connection also deleted.
Now power off the modem, router and PC(s).
Connect ethernet between the modem and router WAN (internet) port - this must be a straight through cable). No other USB or ethernet leads must be connected to the modem.
Connect ethenet between the router LAN (network) port and a computer.
Power on the modem and let settle
Power on the router and let settle
Power on the computer
Check if you can now browse the internet. If not then let us know what happens with the following:
Check that you have connectivity between the various components. The leds should be lit on modem, router and PC card.
Check your PC has an IP address from the router, usually this should be something like 192.168.1.100. What IP has the card got?
Check if you can browse (Internet Explorer) to the router's admin page (check the manual, maybe 192.168.1.1)
Check if you can now browse to the modem admin page, typically 192.168.100.1
On the router's admin page, does the router get an IP address from the Cable network, What IP is is using?