okay most would say that this is a spyware infection and in away they are right,this is related to some kinda peer to peer software program that you may have downloaded such as limewire,bit comet,bearshare etc,they come with such kinda programs that can get pretty annoying,you have a few choices
1>if you are able to identify the program that you download in the ADD/Remove Programs list,try and remove it,anything that looks suspicious
2>as most WOULD suggest download a freeware program that deletes the spyware,(believe me this is a temporary solution theres a good chance it might worsen the situation)
3>do a format and reinstall,i.e reinstallation of the entire operating system,now before you go bonkers over this suggestion consider the pros. if your system was infected with spyware or any virus,its all gone,u wont have any problems atleast for months or years to come depending how you use your computer,you will start afresh which means you will know what you are downloading,performance of the computer will be good,there is cons as well that YOU WILL HAVE TO BACK UP DATA n all that....but considering the problem you have eventually it would land up to this option sooner or later.....so i would suggest if you would consider the reinstallation,but before you go ahead PLEASE BACK UP ALL DATA BEFORE PERFORMING THE FORMAT AND REINSTALL for the operating system
THIS IS A SOFTWARE PROBLEM AND NOT RELATED TO YOUR INTERNET
also try and see whether you can use the internet explorer with out add ons, click start-->all programs-->accessories-->system tools-->internet explorer(without addons)
if you can use the internet explorer,
1>reset the internet explorer(the one u normally use)
2>in the internet explorer click TOOLS-->INTERNET OPTIONS-->ADVANCED-->UNCHECK the option that says "Enable third party browser extension" click apply and ok
see if it works
also have you tried a SYSTEM RESTORE to the time it WAS working fine???