Unfortunately, I think that you are pretty much out of luck using that STB tuner with Win2k, UNLESS you can find a driver on 3dfx's website under Legacy Products. But seeing that 3dfx is now being liquidated....well, it's probably time to start considering alternative options (i.e. buy a new tuner card).
🙁
I know for a fact that the Hauppauge ones work (my friend has one), but avoid ATI tuners at all costs; it doesn't seem like they'll ever learn how to write a decent Win2k driver.