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Did you do a repair of avast via add/remove? What previous antivirus did you use prior to avast? Did you use it’s uninstall utility?

First do a repair? If it doesn’t work. Do this following:

  1. If you used a prior antivirus prior to avast run it’s removal tool in safe mode.
  2. Uninstall avast via add/remove and follow the instructions.
  3. Download and run avast uninstall utility and let the tool run your computer automatically in safe mode. Follow the instructions.
  4. Install a fresh copy of avast. Once installed it will do a quick start up scan. Once finished restart your computer manually
  5. Register your copy of avast.