Can you tell me (us) what version of Adobe Acrobat you're using? Also, what version of Adobe Reader as well?
Also do note that if you have Adobe Acrobat installed, you don't need Reader.
I just installed Adobe Acrobat 9 on W7 x64 and during installation was asked which program, Reader or Acrobat, I wanted to use as a default for opening PDF files. I chose Acrobat.
Hope that helps.