Sounds like an infection to me? There are some infections that disable anti-virus software. Are you getting redirects when you open your browser especially if you try to download any anti-virus, anti-malware programs etc? If so, you are infected! If you’re not experiencing any other suspicious activity like I mentioned above I would still suggest a free online scan if only for piece of mind. Once you are sure you’re not infected you can attack it from another angle.

First right click Avast icon in system tray, select “Avast! Shields Control” then click “Disable Permanently” Don’t worry, you can re-enable it later.

Go here and run a free online scan> http://www.eset.com/us/online-scanner/ Check the boxes shown in the graphic below before you start the scan. When the scan is finished go to C:\Program Files\Eset folder and copy the contents of the “Log” text file here in this thread. Re-enable Avast shields before attempting any online activity.