Internet connection

It will help you to disclose the type of connection and involved to your computer. Assuming that the connection done with a wire from the computer to the Modem. Check the Device Manager for valid Wire card entry. If there is No valid entry, delete any Bogus Entry and re install the Wire card Drivers.

Check the Network connections to make sure that you have a Local Area Connection Icon Entry and that the Properties of the Icon are correctly configured with TCP/IPv4 protocol in the Network Connections Properties. Log to the Router's menu from a working computer and make sure that the DHCP is On and it is configured to assign IPs that can cover the number of computer that are connected Switch Off the computer  and the Router, Reboot the Router, and the computer. Right click on the Wire card Network Connection, choose Status, Details, and see if it got an IP and the rest of the settings. Also make sure that the Software Firewall on each computer allows free local traffic. If you use 3rd party Firewall On, Vista/XP Native Firewall should be off, and the active Firewall has to adjust to your Network IP numbers on what is some time called the Trusted Zone.

General example, 

Please Note that some 3rd party Software Firewalls keep blocking aspects of Local Traffic even if they are turned off. If possible configure the Firewall correctly, or totally uninstall it to allow clean flow of local network traffic. If the 3rd party software is uninstalled or disables, make sure that Windows native Firewall is On. If the above did not help. From a command prompt type ipconfig /all >Desktop\Ipconfig.txt and press Enter. Paste the content of the Ipconfig.txt file that is on the desktop onto a post. "Windows could not start diagnostic policy service on local computer. Access denied" One of your recent system updates has caused this problem.