如下是彈出的原因：
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.
大致是說HTTPS傳輸的內容是加密的，HTTP不是，所以在HTTPS的網站上如果HTTP的內容，那麼IE8就會彈出warning。
如下操作步驟：
Instructions
1 Click "Tools" and "Internet Options" in the Internet Explorer window. 打開IE，選Tools->Internet Options.
2 Click the "Security" tab at the top of the "Internet Options" window. 選擇Security選項卡
3 Select a zone for which to disable the security warning, such as "Internet," by clicking its name. 選擇一個需要去掉warning的zone，一般爲Internet
4 Edit the security settings by clicking the "Custom Level" button. 開打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選項
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. 選擇Enable，默認爲prompt
7 Save your settings by clicking "OK" twice. 最後選擇OK保存
原文地址：http://www.ehow.com/how_6899569_disable-ie-8-security-warning.html