My internet connection is now running slow. Help!?

A few days ago, my internet connection is running at dialup speed. I have an optimum online modem and my Windows xp computer is connected to it via a router but my windows 7 computer with the same modem is running slow as is the other one. I have ran a virus scan with malwarebytes and used cc cleaner to fix any errors, restarted comp. and deleted cookies etc but it is still at dial up speed, help please!