One of the possible causes is a process that is interfering. Did you install any programs just before the problem started to occur? I also suggest you disable all startup programs (except the AV) and see whether that helps. If yes, then you can try to isolate the culprit.
Another possibility is an add-on in the browser. If you are using the IE, run the IE without add-ons for a test. You find that in All programs > Accessories > System tools.