Open up Windows Media Player, right click somewhere on the player\'s window, choose Options, and go to File Types to change what WMP will automatically play.
If that doesn\'t work, open up Windows Explorer, go to Tools:Folder Options:File Types, look up the Quicktime files, click Change or Edit, and change what program you want to open it with. I\'m in XP, so I can\'t do a step-by-step for 9x, since it\'s a different setup.
Though, I\'m surprised you were able to run Quicktime files with WMP. I don\'t think it\'s ever worked for me.