Check your My Documents folder for a file (not a folder) called
"Annotations." Reader is supposed to create a folder in My Documents called
Annotations to hold bookmarks and annotations for each book you read. On my
WM6 device (an AT&T Tilt), Reader would inexplicably create a file by that
name instead of a folder, causing Reader to hang when opening a book (it
tried to create the Annotation file for that book but there was no folder to
write it to.) It's probably trying to delete this non-existant folder when
you attempt to uninstall.
Deleting the file named "Annotation" and manually creating a folder by that
name fixed the problem for me. I think the lasted version of Reader,
released a month or two ago, also solved the problem.
In my case, I've ditched Reader for Mobipocket, leaving all Reader problems
behind me... (The incompatibility with the Reader plug-in and Word 2007 was
the last straw for me!)