Fixing the IE 8 warning - \'Do you want to view only the webpage content that was delivered securely?

We have 2 websites. Each displays the IE8 default warning "'Do you want to view only the webpage content that was delivered securely?" when the customer goes into the shopping cart. (see attached screenshot) We would like both the websites checked for all HTTPS pages accessing embedded resources that are not using the HTTPS protocol, or any other causes for this warning being displayed. Also we would like the underlying issues that cause this to be fixed on both websites so the warning is not displayed to customers. Both websites are Zencart websites.