thanks guys! i'll try the quicktime thing first and if that doesn't work, i'll start messing with audio and video acceleration.
i know how picky the old games can be about quicktime... i think the only reason i got riven DVD running in the first place is because my computer was brand new and i didn't have any quicktime installed yet. as soon as i saw that x-files required version 3.0, i thought it was going to be trouble...
EDIT: looks like it was a quicktime problem, after all! i uninstalled quicktime, deleted all the quicktime files left on my machine, and then reinstalled the version that came with the game, and voila! i'm good to go! thanks so much, i'm really enjoying it so far! (they just don't make games like they used to...)