Did you try doing the following below:

  1. Uninstall Avast via Add/remove and follow instructions.
  2. Download avast uninstall utility from here http://www.avast.com/en-nz/uninstall-utility and run it in safe mode.
  3. Download Avast Cleanup Tool from here http://rejzor.wordpress.com/avast-cleanup-tool/. Scroll down a bit and you will find it. Run this tool in normal mode and after the tool is finished it’s job. Restart your computer manually.
  4. Download and install a fresh copy of Avast. After it finished the installation avast will do a quick startup scan. After that is done restart your computer manually again. After restart, register your Avast copy.