Determine the Optimal MAX MTU Setting

There are several programs available that will change you MAX MTU setting for you or you can do it by editing the registry directly
An easy way to determine what that setting should be is to use the PING command.
You basically want to start out with a MAX MTU of 1500 then use the PING command as follows:

PING HOST_IP_ADDRESS -f -l PACKET_SIZE

where HOST_IP_ADDRESS is the IP address of the site you want to ping and
PACKET_SIZE is the size of the MAX MTU setting you are trying to test

Keep decreasing the packet size until you don't get any messages regarding fragmentation

To change the MAX MTU setting, see the section on Changing the MaxMTU and other Network Settings