How to change PPPoE MTU size ?
This tip applies to Windows XP/2k. Using Regedit, navigate to:
In the right, find a value named "ProtocolMTU" and change it to the appropriate MTU for your connection. Reboot for changes to take effect.
Note: You might also have to check/add the "ProtocolType=800" and "PPPProtocolType=21" as per the MS Article below.
See also: MSKB 283165