To permanently associate a file extension with an application, open Windows Explorer and click on the file with the right mouse button ("right", as opposed to "left", not opposed to "wrong").
You will see a drop-down list, with an "Open with . . ." option. This leads to a "Choose Program . . ." option. Select this, and find the application you want. There is a check box with the caption "Always use the selected program to open this type of file". Check the box.
When you click on the OK button, Windows will remember the association, and Robert is your father's brother.