Assuming you are in windows, right click a file with the extension you want to open in wmp 6.4, and click "open with", find the program you wish to use, (if its not on the list you can find it browseing for the file. This works with WINXP and server 2003. For windows 98 you go through folder options/properties... I am not 100% sure, I might update this once I get a chance to get on win98... (this computer isnt dual booted)