To associate a file extension with a program : (All Windows versions vary slightly - XP shown)
From your windows control panel select "Folder Options" and click the "File Types" tag
Click the "New" button.
Enter the file extension and click "OK"
The new file extension appears in the list. Now click the change button.
If the file extension is known to windows, your program will be selected. If not, you will see this dialog:
If your program is in the list, select it. If not, click the "Browse" button.
Windows displays the standard file selection dialog.
Locate your program and double click it.
The program now appears in the list:
Double click it and :-
Windows now associates the program with the file extension.