When you install an application in Windows, the application's installer usually associates the application with the file formats commonly used by that application. A file in a format that's associated with an application then appears with that application's file icon. When you double-click the file, the application automatically starts and opens the file.
For example, when you install Adobe Photoshop in Windows XP, the Photoshop installer associates Photoshop with image file formats, such as TIFF and JPEG. If you later install another application that commonly uses the TIFF or JPEG file format, files in those formats may then be associated with the new application.To change the association back to Photoshop or to a different application, reinstall Photoshop or the application with which you want the files to be associated.
You can also manually associate a file format with an application.
To manually associate a file format with an application: