OK, I'm going to chime in on what solved this issue with me. I have Avast Internet Security on my business computers. All computers were down. I attempted update of definitions, but Avast told me I had the most recent definitions. I spent 2 hours scanning the computer, which "detected" 4,677 "infected" files, which were ALL html files. Being a web designer, of course I have HTML files, but when Avast told me that html files for Quickbooks and Adobe CS4 were infected, I was HIGHLY suspicious. I was almost ready to install another internet security suite, but decided to update the software. Updating the software from Avast 5 to Avast 6 resolved the issue.
If updating the definitions doesn't resolved the issue, open the Avast control panel, go to Maintenance, and update the Program. You will likely be asked to reboot your computer (it did for me). You should be updated to version 6, and should be able to access all websites again.
Apparently, there was some glitch in the recent Avast upgrade that blocks html pages, which is, of course, pretty much every web site on the internet.