Long ago, when I first heard of the dynamic tuning in the Windows Vista TCP/IP stack, I envisioned the manual setting of the static MTU to be a thing of the past. Unfortunately, I was wrong - the MTU is still a fixed setting, in Vista.
The legendary tool, DrTCP, which is used by everybody to change the MTU in Windows 2000 and XP, doesn't work under Windows Vista. Fortunately, Microsoft now allows us to adjust the MTU using the "netsh" command. As other commands in Windows Vista, you run "netsh" using the command window, in Administrative mode.
To see what interfaces you have on your computer, type
netsh interface ipv4 show subinterfaces
To change the MTU, type
netsh interface ipv4 set subinterface "Local Area Connection" mtu=nnnn store=persistent
Local Area Connection is the name of the network connection on your computer, from the list obtained above.
nnnn is the desired value for MTU.
Reboot after making the change.