User Interface Privilege Isolation (UIPI) is a technology introduced in Windows Vista and Windows Server 2008 to combat shatter attack exploits.
By making use of Mandatory Integrity Control, it prevents processes with a lower "integrity level" (IL) from sending messages to higher IL processes (except for a very specific set of UI messages).
[1] Window messages are designed to communicate user action to processes.
However, they can be used to run arbitrary code in the receiving process' context.
[3] Microsoft Office 2010 uses UIPI for its Protected View sandbox to prohibit potentially unsafe documents from modifying components, files, and other resources on a system.