This problems usually caused when your computer cannot renew/obtain it's IP address.....try repairing your network connection, Start,Control Panel, Network Connections, right click on the connection and click repair. Doubt this will work but its worth ago. Second option is to disconnect the power cable on your cable modem for a minute then reconnect, this usually solves the problem. Third and finally option would be to assign an IP manually. Start by running the command prompt, type ipconfig, it'll display your IP. Next go to network connections (Control Panel) and right click for the properties, once open go to TCP/IP and click properties enter the IP address thats displayed on your DOS screen, put the subnet mask to 255.255.240.0, next in your system tray you should have a little yellow triangle with an explanation mark on it (Your limited or no connectivity icon) click this and it should display more details, in there you'll find your default gateway and DNS IP's as provided by your ISP, enter these in the relevant bits on the TCP/IP properties.
Restart your PC and you'll find that the problem will be fixed. Leave Norton on, theres nowt wrong with it.