I am having the exact same problem as others on this thread. Certain websites that were working perfectly fine a few days ago are now not work. Examples are the bank of america site, apple.com (but, curiously, not this site), the firefox site, and other completely random sites. I can go to gmail, facebook, and others just fine.
I have tried all the common things mentioned here and elsewhere (clearing cache, creating a new account, etc.), and the only thing that has worked is using a proxy site, but I don't want to do that forever, especially for things like online banking.
Others have fixed this problem by uninstalling something called peerguardian, but I do not have that on my computer, so that is not the problem. Someone please help!