Download these 2 tools and run them to remove Avast and clean up all the components.

Then try a fresh install of the trial version.

Run them in this order :

  1. http://files.avast.com/iavs9x/avastclear.exe

  2. https://skydrive.live.com/?cid=94a12102e5094675&id=94A12102E5094675!952&authkey=!AHu_KR3cJN1jHt4