Firstly, Vista does auto tune RWIN but is dead twitchy and conservative, which is why it sometimes switches to 'Highly Restricted' for the session during boot or some minor glitch later on in the session and then won't switch back automatically with ease. It is for this reason that the occasional check with my script is very useful for Vista so you can switch back to 'Normal' if it goes quirky. Having a setting of 'Highly Restricted' will really slow you down, and this solves a lot of Vista speed complaints. It is a pain to baby sit, but the solution seems to be Windows 7 which has not done this to me yet at all.
The settings available for the auto tuning level are as follows:
disabled: Fix the receive window at its default value of 65536 bytes which you can't then change as far as I can tell except by choosing one of the following options.
highlyrestricted: Allow the receive window to grow beyond its default value, but do so very conservatively.
restricted: Allow the receive window to grow beyond its default value, but limit such growth in some scenarios.
normal: Allow the receive window to grow to accommodate almost all scenarios.
experimental: Allow the receive window to grow to accommodate extreme scenarios.
WARNING: This can dramatically degrade performance in common scenarios and should
only be used for research purposes.
Use the Tweak Test from the following site to guage the effects:
http://www.dslreports.com/tools
Have fun experimenting with that last setting
~