Internet Explorer cannot display the webpage

I am unable to bring up any web site using IE. Firefox works fine. It started happening after installing April Security update. I have been working with Firefox so far. But I have to use some sites which only support IE. I tried uninstalling IE7 and re-installing it. It didn’t help. I tried running IE without any add-ons. It didn’t help either. Any help in diagnosing and fixing this will be much appreciated.