The Adobe Acrobat/Reader that is running cannot be used to view PDF files in a web browser

This is caused by an incorrect registry entry.

From: http://kb2.adobe.com/cps/405/kb405461.html

Go to Start > Run
Type regedit and click OK.
Browse to the following key:

HKEY_CLASSES_ROOT\Software\Adobe\Acrobat\Exe

Make sure that the correct Default data value is set for the path where Adobe Reader or Acrobat are installed.

The default installation values are

"C:\Program Files\Adobe\Reader 9.0\Reader\AcroRd32.exe"

or

"C:\Program Files\Adobe\Acrobat 9.0\Acrobat.exe"

depending on whether Adobe Reader or Adobe Acrobat is installed.