Navigate the the location below in your registry and make the corresponding change.

Add or edit the existing value named "MTU" size to equal the desired size. For modem connections a MTU of '576' is recommended.

Key:[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters]
Value Name: MTU
Data Type: REG_DWORD
Data:Default = 0xFFFFFFFF