If you use Internet Explorer 9, it is set to Prompt by default. When this setting is set to Enable, Internet Explorer does not prompt you with the “Only secure content is displayed” message even if the webpage is using unsecure HTTP elements.
The reason for the secure content message in Internet Explorer 9 is to help protect you from security threats. The warning is letting you know that you are on a secure (HTTPS) page that is attempting to load non-secure (HTTP) content. You can just disable it as JThompson stated. It works.