Associating an application to a file extension

By default, the Windows operating system displays only the file names along with the icons of the associated application which is used to open the file. For the files that the Windows operating system does not recognize, the full filename including the extension is shown. The Windows operating system provides users with the choice to change the selected program to open a particular type of file. It will show a list of recommended programs and other related applications installed on the system for the user to choose from. Users can also associate an application to a file extension.

Display all file extensions

Windows XP by default will mask Extensions of known type. To view all extensions, from My Computer or Windows Explorer, click the Tools menu and choose Folder Options, then click on View.
Turn off the box Hide extensions for known file types then Apply.

Modify an existing file association

If a file extension is already assigned to a program it is possible to change the association
Follow the steps below:

Right-click on a file with the extension whose association must be changed, then click Open With.
In the dialog box opened, choose the program with which the extension must be open (or Browse to select a program of your choice).
Check the option Always use the selected program to open this kind of file and then confirm by clicking on OK.

Create a new file association

If the extension is unknown to Windows, it is possible to create a new file association.
Right-click on a file.
Click on Open, or double-click the file.
Windows XP displays a dialog box informing no program is associated with this file. The dialog box open and offers the following two options:

Use the Web service to find the appropriate program.

Select the program from the list.

Choosing the second option. In the dialog that opens, select a program or click Browse to navigate until you find the program corresponding to the extension.
Check the option Always use the selected program to open this kind of file and then confirm.
The file will now be associated with a program.

Manage the list of file extensions

From your Windows Explorer, it is possible to modify, create, delete, change the icon associated with the type of files, or restore the type of files by default.
Click on Tools and then Folder Options.
Select the File Typesoptions
The list of extensions, the type of files as well as the program associated to them will be displayed.
Select the one you want to modify.

If you are using Vista, you can follow the same steps!