Secure and Nonsecure items

What constitutes “nonsecure” items on a web page?

I have a webpage that is hosted on IIS 5.0 with .net Framework SP2 and from the very root of the site IIS is set to require SSL. It all is fine at the root but one folder level down I have a portals aspx app and when that page loads I get the “this page contains both secure and nonsecure items” warning. The page contains links to http:// pages of external sites but no graphics are being loaded from anywhere other than my secure site and it’s sub folders.

What could be the cause?