How to check MTU size under Vista ?
To check your Vista MTU settings, open command prompt as administrator, and type the following command:
netsh interface ipv4 show subinterface
You will be presented with a list of active network interfaces similar to the following:
MTU MediaSenseState Bytes In Bytes Out Interface
Note the corresponding MTU value in the first column.
See also: How to set the MTU size under Vista ?