Windows associates file extensions, such as “.doc” and “.xls”, with programs that operate on files of such extensions. For example, files with extensions of “.doc” might be associated with Windows WordPad. Then, when you double click on a “.doc” file in Windows Explorer, WordPad opens the file.
To see associations, open Windows Explorer and click its menu for: View | Options | File Types. Or click on “My Computer,” an icon on your desktop (Windows’ main window), and a window captioned “My Computer” will open. Click the menu for: View | Options | File Types.