Maybe the download file was corrupt?
Back up your Settings, Ahterr, Skins and (if you got custom ones) Sounds/Sights folders. Then uninstall AH. Download the current full version from the homepage and reinstall. Start the game. If it works, quit it again and place your backed up custom folders back in you new installement.
PS: Its good to keep a backup of your settings somewhere. In case you ever loose the install, you dont need to set up stick and all head pos etc again.