Acrobat 9 does not display PDF in browser

This could be a problem after upgrading Acrobat Reader or running multiple versions of Acrobat. In my case, I have Acrobat 7 and Acrobat Reader 9 installed on the same machine.

I changed this key to 1 and it is fixed.

HKCU\Software\Adobe\Adobe Acrobat\7.0\Originals\bBrowserIntegration

http://kb2.adobe.com/cps/406/kb406119.html