The Windows system control, Explorer UI, Internet Explorer UI, Office UI all use undocumented APIs since Win98/2k and Office 95.
It's called Windows "DirectUser" UI and it's in violation of the antitrust settlement.
sources:
* http://blogs.msdn.com/b/oldnewthing/archive/2006/02/10/52952...
* http://social.msdn.microsoft.com/Forums/windowsdesktop/en-US...
Internet Explorer's use of DUSER.DLL is only temporary.
The final version of Internet Explorer will not use it.
-- Raymond Chen [author of "The Old New Thing"]