This step-by-step article describes how to change the default action for a file type in Windows XP.
The default action for a file type is the action that occurs when you double-click the file. The file type is determined by the file's extension, and all files that have the same extension have the same default action. The most common default action for document files is Open, which launches the associated program and displays the file in the program's window.
NOTE: To determine the default action of a file type, right-click the file listing. The command that is shown in boldface on the shortcut menu is the default action.
For some file types, the default action may be undesirable, or even dangerous. For example, a registry file (.reg) has the default action Merge, which sends the data in the file to the registry, and replaces existing registry values or adds to existing registry values. The Edit command is available on the shortcut menu for a .reg file, but it is not the default action. If you want the default action for a .reg file to be Edit, and require that users right-click to view the shortcut menu and deliberately click Merge, you can change the default action for all .reg files. Similarly, you may prefer to have the default action for a batch file (.bat) changed to Edit instead of Open. Double-clicking the file will not run the commands in the file, and if users want to run the file, they can use the Open command on the shortcut menu.