Get 401 Unauthorized error for images and js files

Hi,
I get 401 Unauthorized error for most images, CSS and js files in my ASP.NET web application. IIS_IUSRS has read & execute permissions on application's folder.
Only content in subfolders MyWebApplication\Content\img and MyWebApplication\Scripts seem to be affected.
In the browser the dynamic content of the web application is loaded properly while most (not all) of the static content is missing.
Any idea?
Thanks