File Association and the DropTarget Reg Key

When setting File Association for TIF files I encountered a strange problem.

I set the basics for file association (i.e)

HKCR\.TIF  tifimage.document
HKCR\tifimage.document\Shell\Open\Command = "C:\myviewer.exe" "%1"

But TIF Files kept opening in a viewer other than the myviewer.exe specified.
Inside the Open Registry Key was another key called DropTarget
Renaming/Deleting this key fixed the issue.

Im not to sure on the details of this key but it was not required in my environment.
Details on the key are
http://msdn.microsoft.com/en-us/library/ee872121(v=vs.85).aspx