In tip 0108, I described how to alter the Open association from a CMD line by using the Assoc and Ftype commands.
Microsoft Knowledge Base Article 320033 contains the following summary:
This article describes how to change which program is used to open a file in Windows 2000. For example, you can specify that files with a certain extension open automatically in a specific program. You can also modify an existing file type to change its association with a program.