results in 'It works' then there is a chance of 99% there is another webserver running, because a XAMPP installation doesn't give that page by default when accessing that page. It's very likely the built-in webserver that came with Mac OS X has been accidently turned on.
Check System Preferences > Share and look if 'Webserver' has been turned on. If so, turn it off. Then start XAMPP.