I think you are experiencing what happened to us this past weekend! Do you only have antivirus, or a firewall too? We found that the firewall gave us some problems. Disabling it allowed us to get to other sites, but of course, our computer wasn't protected!
Just this morning we figured out how to solve it so that it was enabled but we could still browse. I'm not at home right now though, so I can't tell you how we fixed it!