Each file type has a program associated with it that will be used by default when opening the file. The easiest way to change a default program association in windows versions other than XP is to shift-right click on a one of the files of the type that you want to change and select Open With from the menu.
In Windows XP you right click on one of the files that you want to change and select Properties, then on the General page you select Change to bring up the program selection list.
You can now select the new program that you want associated with this file type from the list or use the Other button (Browse in XP) to browse to find the program. Next, check the Always use this program box, and then press the OK button. This file will be opened by the selected program as will all files with the same file type that you open in future.
You may find that the entries listed in the dialog box don't make any sense to you (for example I have no idea what "org5 is). If you can't identify the program that you want to associate with the file from the list then click on the Other button and navigate to the location on your system where you installed the program and select that. The entries in the open with dialog are just shortcuts to some of the programs that you have installed on your system to make it quicker to set up associations.
This article written by Stephen Chapman, Felgall Pty Ltd.