As to what is causing this problem, only Adobe would be able to tell you. Try to uninstall Reader, then run Adobe's Cleaner tool (http://labs.adobe.com/downloads/acrobatcleaner.html), and then install from a fresh download again. I've seen a number of people complain about this problem, and a re-install after cleaning the system from all traces of Adobe Acrobat or Reader should do the trick.
Karl Heinz Kremer
PDF Acrobatics Without a Net
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