如下是弹出的原因：
Internet Explorer 8 displays a security warning box when you're on a secure, HTTPS-protocol-using website that at temps to load non-secure content over the HTTP protocol. HTTPS content is encrypted so no one can snoop on your personal information, but HTTP is not. HTTP content on an HTTPS website can cause sensitive information such as credit card numbers to be sent unencrypted, so IE 8 displays a security warning. If you realize the risks and don't want to see this error message every time a website loads non-secure content on a secure Web page, you can disable the security warning.
大致是说HTTPS传输的内容是加密的，HTTP不是，所以在HTTPS的网站上如果HTTP的内容，那么IE8就会弹出warning。
如下操作步骤：
Instructions
1 Click "Tools" and "Internet Options" in the Internet Explorer window. 打开IE，选Tools->Internet Options.
2 Click the "Security" tab at the top of the "Internet Options" window. 选择Security选项卡
3 Select a zone for which to disable the security warning, such as "Internet," by clicking its name. 选择一个需要去掉warning的zone，一般为Internet
4 Edit the security settings by clicking the "Custom Level" button. 开打Custom Level
5 Scroll down until you see "Display Mixed Content" under the "Miscellaneous" header in the "Settings" list. 在Miscellaneous下找到Display Mixed Content选项
6 Click the "Enable" radio button under "Display Mixed Content." This is set to "Prompt" by default. "Enable" causes mixed content to be displayed with no security warning. 选择Enable，默认为prompt
7 Save your settings by clicking "OK" twice. 最后选择OK保存
原文地址：http://www.ehow.com/how_6899569_disable-ie-8-security-warning.html