Doh! Sorry I'm an idiot. I forgot to post the card I'm using.
Yup, it's an SBlive! XGamer or something like that. I found the sblive drivers on this forum for download for Win2K so I got them, but I don't want to screw anything up. I installed Thief in Win98 and it works fine, except there's no sound.
The current drivers for my SBlive are the ones that installed with Win2K.