Have you updated from Acrobat 4?
The only time I have had that is when I updated from Acrobat reader 4 to 5. I did not un-install 4 first. When I tried to open a PDF it tried to open it with Acrobat reader 2.1.
The best thing to do is un-install Acrobat Reader 5.1 and then un-install Acrobat Reader 4 and then re-install 5.1.
Normally you can just over write Acrobat Reader with a newer version as long as the the revision number is the same ie 4.1 to 4.3 or 5.0 to 5.1. You cannot install a new revision number over and older one. ie 5.1 over 4.1.