I noticed a major slowdown in the properties dialog for AVI files on my system and discovered that MSISIP.DLL, a signature handling component of the M$ installer, was scanning the whole files for encrypted signatures!
This file is installed with many M$ patches and third party programs as a part of the installer utility. That it starts to search my media files on it's own when I'm not actually installing something leads me to suspect another sleazy attempt to sneak spyware into everyone's system.
In any case, it slows things down so much that I advise you find that file and rename or delete it. It is not needed for anything until you are actually installing software or one of the many M$ patches and bugfixes, and will be automatically replaced by the installer whenever you need it for legitimate installations.