How do I change my MTU settings?

I have recently been trying to change my MTU settings for my xbox live. I tried going to my routers IP adress and going to change it there but theres no option to change it from there. I have a windows vista pc wireless, D Link router I have tried hooking my xbox up to an ethernet cable but that didn't work I still got the same MTU error. Don't tell me to download Dr.Tcp or whatever that is because as far as I know it doesn't work for windows vista. If there is any solutions to this that may work for me please feel free to post them! :) thanks