Please can someone help here.

No matter what i do i cannot get back onto live,i was on about a week ago and all was fine. i tried to get on today and 360 wont have it, i am not banned(see my prevoius thread) at fist it was saying about my DNS settings so i inputted the ones for my 3 Mobile Broadband(SHITE!)usb modem and now its saying my MTU is too low, this is not right i just been in to change it through the CMD prompt in vista and it is at 1440 which is above what the 360 is asking me for, i am really P****** off with this now, can anyone help,thanks