7777
I have got W2K in front of me at the moment, so the below may be a little different in W98, but not a lot.
Windows Explorer
Tools
Options
File Types
Select the extension you want (or add it, if it's not there)
Advanced
Select Open
Click on Edit to give yourself the idea of how this is constructed
Cancel
New
Action is open
Application is the command line you want executed, including switches
OK
Close
Bob should be your uncle, when you right click you get offered alternatives of want you want to do.
W