Marc Silbey, a Program Manager focused on IE security, disclosed some information about the Protected Mode IE in Windows Vista.
As Rob mentioned, Protected Mode helps to eliminate the silent install of malicious code through Windows Vista’s User Account Protection (UAP) technology by blocking writes outside of the Temporary Internet Files (TIF) folder. Protected Mode also leverages UAP’s User Interface Privilege Isolation (UIPI) to help prevent Window messages from being sent to higher privilege processes.IEBlog
For this release, security is our number one priority and preserving compatibility is a close second priority. To maintain compatibility, Protected Mode launches broker processes at the user and admin privilege levels to accomplish elevated operations like saving web pages and installing ActiveX controls through UAP’s Application Info Service (AIS).