Most likely, a lack of video card
I wasn't a serious gamer when I bought this box, so there's no graphic accelerator (and only 512 Mb RAM).
I'm still not a serious gamer... but I easily could be, to the detriment of the rest of my at-home life.
Odd thing is, my copies of Freespace and Freespace 2 -- and the other mods I've found that require the presence of the Freespace CD in the drive -- all play just fine. That's why I suspect that my root problem is insufficient RAM, even though the game's developers say that 512 is sufficient.