Mike Friedman from the Internet Explorer development team posted an article on IE blog about the new Protected Mode in Windows Vista IE7:
We here on the IE team have a continuing focus on making web browsing safe from a variety of outside threats. Microsoft Windows Vista introduced an enhanced security model that we were able to build on in Vista's version of IE7. I want to tell you about a new major IE defense-in-depth security feature called Protected Mode. Defense in depth is a security principle that a system should provide multiple layers of defense, in case one layer is ever breached. Protected Mode takes advantage of three key new technologies in Vista's security model..