Navigate the the location below in your registry and set the data of TcpWindowSize to the desired value.

Key:[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters]
Value Name: TcpWindowSize
Data Type: REG_DWORD
Data:0 - 0xFFFF (default = 8760 for Ethernet)
More info: http://support.microsoft.com/support/kb/articles/q140/5/52.asp