Microsoft: Reset TCP/IP on Windows

The steps below allow you to reset the TCP/IP on Windows:
1.  To open a command prompt, click Start and then type cmd in the Search programs and files box.
2.  Under Programs, right-click the CMD.exe icon, and then click Run as administrator.
3.  When the User Account Control box appears, click Yes.
4.  At the command prompt, enter the following command, and then press Enter:
     netsh int ip reset c:\resetlog.txt
     Note If you don't want to specify a directory path for the log file, run the following command           instead:
     netsh int ip reset resetlog.txt
5.  Restart the computer.


Reference:
How to reset TCP/IP by using the NetShell utility
http://support.microsoft.com/kb/299357