I think you loaded Win 8.1 on top of Avast 8 which are not compatible. I’d suggest a clean install

  1. Download the right aswclear and the appropriate program

http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe
http://files.avast.com/iavs9x/avast_pro_antivirus_setup.exe
http://files.avast.com/iavs9x/avast_internet_security_setup.exe
http://files.avast.com/iavs9x/avast_premier_antivirus_setup.exe

v8 and below : http://www.avast.com/uninstall-utility
v9: http://files.avast.com/iavs9x/avastclear.exe

  1. Uninstall Avast by control panel [If you don’t have Avast in control Panel go to #4]
  2. Uninstall in safe mode using aswclear (v8 or below) or avastclear (v9) [If you don’t have Avast in control Panel just install it]
  3. Download and install your version
  4. Reboot.

Should all work fine. You can insert license file during the install or after the reboot.