TCP Windows Size vs Socket Buffer Size on Windows
AFD.sys is the library used to support sockets in Windows. To change the send and receive buffer sizes
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Afd\Parameters]
DefaultReceiveWindow = <value>
DefaultSendWindow = <value>
A reboot is required for the changes to take affect.
I've found the sweet spot for 1GbE to be 65536 and personally I think this should be the default instead of the current one which is 8192. RAM is cheap. :D