In the Windows 7 Start Menu type this into the Search bar: file assoc
From the options choose: Make a file type always open in a specific program
This will open a window in Control Panel - Programs - Default Programs - Set Associations
In the list of file types find .pdf and click once to highlight it.
If the program associated is not Adobe Reader choose the Change Program option.
A list of suggested programs should give you the option to chose Adobe Reader.
After you finish the association for .pdf type files click on the Description category in the Name/Description/Current Default header bar.
This will sort all the extensions by program name. Find Adobe Acrobat in the Description list and you'll find other extensions you'll want to change the association: