Security Information alert box on my browser

I have Windows XP and currently using Internet Explorer 7 as my default browser. The Security Information box keeps showing on some websites that I used to visit. But lately, it appears on almost all the site I visit. It is really annoying to click on Yes button every time the page loads. The box content is;
This page contains both secure and nonsecure items.
Do you want to display the nonsecure items?
[Yes] [No] [More Info]
How can I remove it from appearing? Please help.
