Have you installed anything on your pc prior to noticing this sudden problem? Perhaps without really knowing it? The cluprit is often a file downloaded and installed by a 'suspect' website without your knowledge..
Find a program called "HijackIt" and download then run.. it gives you a list of all services and browser plugins on your computer.. often from this you will be able to identify anything that shouldnt be there... and remove i!!
Beware though, this program could potentially allow you to delete required system files, so dont just go bashing that delete button without first knowing what you are doing.. theres plenty of sites out there that will tell you the origin and known use of a file running on your pc.. just take the time to check...
As an alternative, you may just want to check any DNS settings (if you are on a LAN or non-direct Internet connection) and/or proxy settings...