ACKs in Windows

To configure the max outstanding ACKs in Windows XP/2003/Vista/2008/7:

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \Tcpip \Parameters \Interfaces \{Adapter-id}]
TcpAckFrequency = 2 (Default=2, 1=Disables delayed ACK, 2-n = If n outstanding ACKs before timed interval, sent ACK)

More Info MS KB328890
More Info MS KB815230 (XP/2003 needs hotfix or SP2 for it to work)
More Info MS KB935458 (Vista/2008 needs hotfix or SP1 for it to work)
More Info MS KB2020559 (Applies also to Win7/Win2008 R2)

To configure the interval timeout in Windows 2000/XP/2003:

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \Tcpip \Parameters \Interfaces \{Adapter-id}]
TcpDelAckTicks = 1 (Default=2, 0=Disables delayed ACK, 1-6 = 100-600 ms)

More Info MS KB311833 (Win2000 requires SP3)
More Info MS KB321098
More Info MS KB321169

To configure the interval timeout in WinNT SP4 (Go to the Services-key and do a search for "TCPIP" to find the different adapters using TCPIP):

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \{Adapter-Name} \Parameters \Tcpip]
TcpDelAckTicks = 1 (Default=2, 0=Disables nagling, 1-6 = 100-600 ms)

http://smallvoid.com/article/winnt-nagle-algorithm.html