1. Click the Start menu button on the Windows taskbar.
2. Click Run... on this menu.
3. If the computer is holding a current IP address, type 'cmd' (without the quotes) in the text box that appears. A command prompt window appears on the screen.
4. Type 'ipconfig' (without the quotes) to view the status of the computer's IP address(es).
5. If the computer is holding a current IP address, type 'ipconfig /release' to let go of the address.
6. Type 'ipconfig /renew' to obtain a new IP address (whether or not the computer is holding a current address).
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http://compnetworking.about.com/cs/windowsnetworkin1/ht/renewipaddrwxp.htm