I prefer to find the MTU this way,
Finding the largest MTU by EXPERIMENT
First reset windows with Dr TCP to MTU 1500.
Then reset modem/router to MTU 1500 save it, reboot PC and modem/router.
Next from windows, go to Start/Run and type in, cmd (click OK)
At the MSDOS-PROMPT type,
ping -f -l 1472 www.bbc.co.uk
Then reduce 1472 by 10 until you no longer get the "packet needs to be fragmented" error message or