Associate a File Type with a Specific Program

How about Method #5 (for people who like the command line):

1. Open an elevated command prompt.
2. Use FTYPE {fileType}={commandString} to create a file type and associated command to open the file.
3. Use ASSOC {.fileExtension}={fileType} to associate a file extension with the file type you created.

Example:

FTYPE MyCustomType=C:\Program Files\MyCustomProgram\MyProg.exe "%1"
ASSOC .custom=MyCustomType

Note that many file types may already be registered on your system. You can list them all by just typing FTYPE with no arguments.

Method #1 (on the fly):

1. Right-click a file of the type you want to change, and select Properties.
2. In the General tab, click the Change button.
3. Choose a program from the list, or click Browse to select an .EXE file on your hard disk.

Method #2 (view a list of file types):

1. Open Control Panel.

2. In Control Panel Home, click Programs, and then click Make a file type always open in a specific program.

   Or, in the Classic View, open Default Programs and then click Associate a file type or protocol with a program.

3. Highlight a file type in the list and click Change Program.

Unfortunately, neither method in Vista allows you to choose anything but the default programs, such as the programs listed in your files' context menus. If you want complete control over your file types, use this tool:

Method #3 (File Type Doctor):

1. Download and install Creative Element Power Tools.
2. Open the Creative Element Power Tools Control Panel.
3. Turn on the Edit file type associations option, and click Accept.
4. Right-click a file of the type you want to change, and select Edit File Type to show this window:

Source

Adding Method #4:

1. Right click on a file of the type you wish to change
2. Click Open With... (select Choose Default Program... if a sub-menu appears)
3. Select the application you wish to open this type of file with. You can also click the browse option if Windows doesn't suggest what you want.
4. Ensure the box labeled Always use the selected program to open this type of file is ticked