I have Windows XP.
Was able to go online and access sites until Avast performed its latest update early this morning - Thurs Aug 02, 2012.
* Outlook Express still worked and I was able to send and receive email but unable to go to sites.
Note: Some of my emails were stripped of content by Avast which stated the email had a virus (wasn't true because I receive the very same daily emails in 3 different email accts - all from different sites!)
Had to be away from the pc for awhile, hence working on this over 5 hrs after the prob began.
Since I noticed that things changed after the Avast update, I ended up having to do a System Restore and restored my pc to Yesterday's date which removed the new Avast update that was causing the issue.
ALSO, after the System Restore & Before attempting to go online, I made sure to Un-Tick the box in Avast to keep it from automatically updating with the current definition!! (Click on Settings - top right, then Click on Updates - on Left)
Lo and behold, after the system restore, I'm back online and able to access sites again which is why I decided that I had better warn others about this!
Late this evening or sometime tomorrow I will allow Avast to update and am hoping that they fix the current issue asap!
Had this happen once before about 1 1/2 - 2 yrs ago due to a bad update.
Love Avast but like other software companies, sometimes updates get to us before being thoroughly checked out (Microsoft is the worst in that regard!!)