Yeah, thanks for all the help, guys. I've read though all that stuff now, and all it's done is irritate me more, because it's had no effect. There seems to be some exceptions to how well Microchip's drivers will work with Windows 2000 anyways, and part of the irritation I think is that the solutions are beyond even the extensive Microchip fixes for the problem. I know I can't make too much of a fuss - I really need to be using a newer OS, on a much newer computer.
The fact that I was able to get MPLAB 8.10 running on an XP machine to test out my programmer, and I was still having the same problem, led me to believe that it might actually be the 4550 on the programmer that's gone faulty. Blueroomelectronics is mailing me another pre-programmed, so I'll swap that in and go from there.