You get this error because a) you don't have a Default Document set up that matches with a document you have in the folder and b) you disabled directory browsing.
To fix problem a, create a page like Default.aspx. This document will be served whenever the folder (and not a direct page) is requested. To see what default documents your server has, open up the IIS Manager, locate your site and open its Properties dialog. Then click the Documents tab. You see a list with files that will be seen as Default documents. So, when you see Default.html, and you have a file with that name, that file will be served whenever you access something like http://localhost/
To fix problem b (not recommended on a production server as it may expose too much information about your files), once again open the IIS Manager, and open the Properties dialog for your site. Then on the Home Directory tab, make sure that Directory browsing is enabled.
Hope this helps,
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