You may be screwed:
http://support.microsoft.com/kb/935458/en-us
Symptoms
On a computer that is running Windows Vista, you modify the TcpAckFrequency registry entry by following the steps that are described in Microsoft Knowledge Base article 328890. However, the behavior of TCP acknowledgments (ACKs) does not change.
STATUS
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.