Can connect to router but not internet

I have two identical Vista laptops. One connects perfectly to the wireless router and internet. I am using it now. The other connects to the router but has the yellow limited connectivity triangle for about ten minutes before it goes of but for the internet I get the red cross. Clicking to diagnose tries to fob it off on the ISP. I have searched for differences between settings for the two PCs and the only thing I can find that is different is that in the Network and Sharing Center the malfunctioning PC has 'File sharing' set to 'On' whereas the functionning PC has it set to 'Off' and it says my network is an 'Unidentified network' not the BTVoyager2100-B3 that the working one says.

When I tried changing the file sharing setting to mirror the other PC it hung on me and I had to use task manager to close it. The other thing I should say is this all came about when I changed from Norton to AVG. Whole process was fine on this PC but once I uninstalled Norton the other one started playing up. Other big difference is the malfunctioning PC has no antivirus or firewall at present as I was relying on AVG to sort this out but now cannot install it without internet.

I have scanned loads of forums but a lot are trying to put the blame on the router or ISP which cannot be the case here with one PC working perfectly. Any help would be much appreciated.

It is very difficult to troubleshoot a network connectivity problem, whatever the type, without using the network diagram, which shows including IP addresses, roads, etc. IP devices such as firewalls and switches. The logical and physical topologies are extremely helpful in the process of network troubleshooting. Physical network topologies A physical network diagram shows the physical layout of network-connected devices.