Read this and see if it helps. I do not have ECN enabled and I am not experiencing any issues. It is up to you how you want to configure your box.
http://technicallyeasy.net/2010/02/internet-connection-slow-in-windows-7/
This is what I have set for WIN7
Microsoft Windows Version 6.1.7601 Copyright (c) 2009 Microsoft Corporation. All rights reserved. C:\Windows\system32>netsh int tcp show global Querying active state... TCP Global Parameters ---------------------------------------------- Receive-Side Scaling State : enabled Chimney Offload State : automatic NetDMA State : enabled Direct Cache Acess (DCA) : disabled Receive Window Auto-Tuning Level : normal Add-On Congestion Control Provider : none ECN Capability : disabled RFC 1323 Timestamps : disabled ** The above autotuninglevel setting is the result of Windows Scaling heuristics overriding any local/policy configuration on at least one profile.