One of the common causes of the "This page contains both secure and nonsecure items" message is the IFRAME+SSL problem.
If you're using SSL and have an IFRAME without an SRC attribute, the frame will start out displaying a nonsecure blank page which causes the warning. To get rid of the message you need to supply your IFRAME tags with SRC attributes that display secure content from your site.
If you don't know ahead of time what page the frame should display, just make a file that contains an empty html page and use that.
<iframe src="empty.htm"/>