One long-standing issue people have with QuickTime for Windows is its apparent insistence on becoming the default handler for video and images in IE and throughout the Windows environment, despite the setting of file associations to the contrary. Even if you set everything up to open in Windows Media, for some reason QuickTime is still invoked.
This bothers a lot of people and is probably one of the reasons QuickTime for Windows is disliked.
A recent discovery by an associate indicates this is a behavior of Windows’ ‘Set Program Access and Defaults’, which categorically associates web/mail/media with apps which know how to assume the default role. For some reason, Windows associates media playback to the default selection irrespective of any file associations you may have set. Screenshot included.