There is a registry setting that determines what is your default viewer. You can either reinstall Acrobat (the last one in sets the setting) or you can modify this registry key to point to your Acrobat exe file:
HKEY_CLASSES_ROOT\Software\Adobe\Acrobat\Exe
If it's not there create a string value and point it to which exe you want to launch
For example: "C:\Program Files (x86)\Adobe\Acrobat 10.0\Acrobat\Acrobat.exe"
Hope that helps
Paul