Find some software that acts like task manager in xp, btu runs in 98. Check and see how much CPU time it takes when you're playing the video. There's a "system" process in 2k that handles the overhead for things like i/o, might come up that was in 98, too. If you have lots of leftover CPU time while playing, the problem lies elsewhere.
For that matter, the DV codec you're using might be less optimized in 98 than xp...who knows. ----------------------------------------------------------------------------------
...but I'm just a C man trying to see the light