1. Click Start – RUN and type “regedit” and press enter.

2. Navigate to the following Registry Key

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\

HKEY_LOCAL_MACHINE
\SYSTEM
\CurrentControlSet
\Services
\Tcpip
\Parameters
\Interfaces
\

In the right-panel, create a new DWORD named “MTU” with the value (say 1350 in decimal)

NOTE: To create a new DWORD, right-click, select New and select “DWORD Value” and enter the name (MTU here)

From:http://www.windowsreference.com/windows-2000/how-to-manually-set-the-mtu-size-in-windows-xp-2003-2000-vista/