Did you tried opening PDF in browser when Acrobat is not running
You can try and uninstall Adobe Reader from your system and then try instaling it again. While installing, you will get a dialog asking you to chose the default PDF viewing application. Select Adobe Reader in that dialog and continue with the installation. Now, if you try and open any PDF in the browser, it will open it with Reader without any error message.
You can get Adobe Reader 10.0.1 from: ftp://ftp.adobe.com/pub/adobe/reader/win/10.x/10.0.1/en_US/AdbeRdr1001_en_US.exe
Or Adobe reader 9.4 from: ftp://ftp.adobe.com/pub/adobe/reader/win/9.x/9.4.0/en_US/AdbeRdr940_en_US.msi