1) Download this file: http://www.carricksolutions.com/XP_MTU.reg
2) Right click the file and choose "Merge"
3) Click "Yes"
4) Go to "Start" -> Click on "Run" -> Type in "Regedt32.exe" -> Click "OK"
5) Go to this Reg Key "[HKEY_LOCAL_MACHINE\ SYSTEM\CurrentControlSet\ Services\NdisWan\ Parameters\Protocols\0]"
6) Double click on "Protocol MTU"
7) Click on "Decimal" -> Enter your desired value (I.E.: Anything less then 1480.)
8) Click "OK"
9) Done!