please start Windows Explorer and enable viewing hidden files AND protected operating system files in the Windows Explorer options. Then check if there is a file called "Program" (not a directory, but a file) directly located under C:\ on your hard disk. If there is such a file (not a directory and only if the file is exactly named "Program"), delete it, run the avast! Uninstall Tool, reboot and try to run the setup again. That should work.