There are several ways of changing file associations, the easiest probably is to right-click on a PDF file and select 'Open with...', then select Adobe Reader and check 'Always open...'.
Thanks for the suggestion, but I've already tried this and Reader isn't a choice. When I browsed for a program, I can find Reader but cannot choose it. I don't understand this at all.
We had a very similar topic recently (http://forums.adobe.com/thread/691994), but it is not clear to me how exactly the user resolved the problem - can you have a look at it?
The second method would be to open Windows Explorer, then click Tools | Folder Option, tab File Types. Select PDF, then change it to the program you want to use to open PDF files.
Again I don't know if this still works the same way in Windows 7.
The last suggestion would be to uninstall, then reinstall Adobe Reader - this should automatically associate the PDF extension with Adobe Reader.
Thank you very much for the link. This user says his friend helped him by
configuring Regedit. The Registry does show AcrRd32.exe as an "open with"
choice. Unfortunately, I can't figure out how to use this info as I don't
understand the Help file. Any help available?
The registry entry should contain the full path to AcroRd32.exe, e.g. "C:\Program Files\Adobe\Reader 9.0\Reader\AcroRd32.exe" "%1"
If you are not comfortable to edit the registry, it would be better if you uninstall & reinstall Adobe Reader.