I came across this issue recently while trying to install the latest version of Adobe Reader X (10.1).  Once Adobe Reader X was installed, I was unable to view embedded PDF documents in Internet Explorer 8 on Windows 7 x64.  I did a little research and found out that Adobe Reader is 32-bit only, so you are supposed to use Internet Explorer 8 32-bit.  No problem.  I switched over, but to my avail, I was still unable to view embedded PDF documents.  A little red x would show up in their place, meaning the Active X plugin had failed.
» More: Adobe Reader X – Red X with Internet Explorer on Windows 7