Unfortunately traditional MTU tools such as Dr TCP wont work on Windows Vista and if you wish to change your MTU settings these have to be configured manually.

Step One ~ How to check the your MTU setting in Vista.

From the command prompt (start, run, cmd), type the following command:-

netsh interface ipv4 show subinterfaces

This will show the MTU set by vista on each network device.
As you can see from the image below that even though MTU of 1430 is set for best performance on my router, that vista still has the default windows setting of 1500 on both the wireless and ethernet network cards - this is despite the auto tuning feature that is supposed to automatically adjust the MTU figure.

We can get more information about the attached network devices by using the command

ipconfig /all

If you are going to be changing your MTU settings make a note of the exact names of the network devices. In the example above they are the words directly after adapter and what Ive highlighted in yellow ie "Local Area Connection" or "Wireless Network Connection".

Step Two ~ How to change your MTU setting.

First of all you can only change the MTU value with elevated privileges. Even if you have administrator privileges you will still need to run cmd as Administrator. If you miss out this step then you will get the error "The requested operation requires elevation".

~ Run Cmd as Administrator

Either:

From Programs > Accessories > Command Prompt >
Right Click on Command Prompt > Select "Run as administrator"

OR:

Navigate to the command prompt in C > Windows > System32

locate the cmd icon and right click.
Select "Run as administrator".

This will open a new cmd window in which we can configure windows settings.
You should see the new cmd window is headed "Administrator" rather than your user account name.

Step Three ~ Setting the MTU value

Use the following command to set the MTU value:

netsh interface ipv4 set subinterface "Network Connection" mtu=xxxx store=persistent

replace "Network Connection" with the name of the network device you made a note of above, and replacing xxxx with the new MTU value.

In my example I used netsh interface ipv4 set subinterface "Wireless Network Connection" mtu=1430 store=persistent

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RWIN and Vista

Unfortunately there isn't any way to manually configure the RWIN settings in Vista - unlike previous versions of windows, it doesn't have a registry variable for "TCPWindowSize" that can be tweaked.
Vista uses TCP Auto-Tuning and therefore is supposed to automatically adjust the TCP window size for the best performance for a particular connection.

For more information about Vistas Auto-Tuning see Vista TCP tweaks under Receive Window Auto-Tuning