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To change your MTU to 1400, download DrTCP to your desktop from here.
Set your Dial-up Adapters' MTU to 1438 from the Adapter Settings list. If you use ICS, then you will also need your MTU set to 1438 under ICS MTU. If you also use a NIC, then set that to 1438 under Adapter Settings as well.
Click Save, Exit and reboot your PC. Run a tweaks test to confirm that your MTU is now 1400.
The reason behind the 1438 MTU value is that there is an awkward 38-byte overhead, which is deducted from the actual value you entered on your dial-up adapters.
In other words, 1438-38 = 1400 -- your actual MTU result.