i created chaos on my system trying to manually uninstall some broken software. one thing that seems to have stopped working as a result is viewing pdfs in the dopus viewer pane. it now tries to open with internet explorer. i found an old post saying to reinstall acrobat if this is happening, but since it was working fine yesterday i thought i would make sure this is the correct solution.
Reinstalling Adobe Reader will probably fix it, so give that a try.
If it's still broken after that, see if other programs are able to view the file. e.g. If you double-click it or drop it on IE, does it show up?
If you're on 64-bit Windows, make sure the PDF-ActiveX plugin is disabled and make sure you're using the latest ActiveX+Preview+Office+Web plugin. (You can do that on 32-bit Windows as well, but it isn't vital like it is on 64-bit.)