ASP.Net for the most part will not correctly interpret classic ASP file. If you have a very simple classic ASP page, with perhaps only a few very simple server expressions, such as a <%=Date() %>
or two, you might get very lucky and find that the code still works... but I wouldn't count on it.
You can have classic ASP pages live in the same IIS web site as ASP.Net pages, but you still need to have those pages configured so that the classic ASP interpreter is responsible for them. You can also configure authentication to happen at the IIS level, or configure ASP.Net and ASP classic to share an authentication token/cookie, so that a user logs into an ASP.Net site once and the classic ASP pages are still protected by that single authentication event. However, getting this set up and working correctly is not an easy operation.