Did you install avast in the default location or choose one of your own ?
Sometimes if you chose a your own location the folder name or one in the path has accented characters and this could cause a problem.
So if you try the reinstall Ensure you use the default location.
1. Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
2. Download the avast! Uninstall Utility,
find it here and save it to your HDD.
3. Now uninstall, reboot.
4. run the uninstall utility, reboot
5. install the latest version, reboot.