Using Windows 7 x64 and Internet Explorer, I'm having problems when reading a pdf in the web browser, with Acrobat X. When I close the tab or IE9, the following message appears: "Internet Explorer has stopped working..." without giving any more detail about the failure. I have tried disabling all the plugins in Internet Explorer (using IE with no complements) and the failure is still there. The only way to avoid it is to disable IE9 and Acrobat integration and to open the pdfs directly in Acrobat, not in IE9.
After uninstalling and trying with Acrobat 9, everything seems to go fine.
I have the same problem with Acrobat 9 on IE9.
I have no problems with Acrobat 9 or Acro X on IE8.
So, the problem seems to be specific to IE9.