Do you know what codec you used? If it's Windows Video 9, or if it's protected then there is no way to play the video on the Mac unless the user first downloads something like Flip4Mac (like BB-Rick mentioned).
If you've encoded in WMV9 and need to therefore re-encode, you might consider On2's Trumotion VP6 (which is in Flash, so virtually everyone can play it).
http://www.on2.com/technology/vp6/