First try a known good CAT5 or CAT5e cable that is not a crossover cable.
Open the Device Manager and expand Network Adapters. If the Ethernet adapter is flagged with a trouble icon, double-click the device to view the error code, problem description, and a proposed resolution method.
If the Ethernet adapter appears to be functioning properly, open the Network Connections folder, right-click the connection, and click Properties. Double-click Internet Protocol.
Select the automatic IP and DNS server address options, then click OK and OK. You may need to disable and reenable the connection for the changes to take effect; right-click the connection for these functions.
If these is no connection, open an elevated command prompt —
http://pcsupport.about.com/od/windows7/a/elevated-command-prompt-windows-7.htm
— then type these commands, pressing Enter after each, and restart the computer when done.
netsh int ip reset >NUL
netsh winsock reset