The recommended receive window should also be an even multiple of the TCP Maximum Segment Size (MSS). 

Navigate the the location below in your registry and make the corresponding change.

Create or edit the existing DWORD value (string for Windows 98) named "DefaultRcvWindow" and set it to equal the desired window size in bytes. 

Restart Windows.

Key:[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\MSTCP]
Value Name: DefaultRcvWindow
Data Type: REG_DWORD
Data:size in bytes (Default = 8192)