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.