Posr runs under all 3. I've had all 3 and used it under all. I had some problems with w2k. basically it erased all the files in its own window directory. As for NT, supplies are limted meaning its getting harder and harder to actually get a boxed version. Poser doesn't care really what platform its on. There's nothing win98/95 specific. The only problems I did have were with the file type loaders. When I installed w2k, I lost the ability to load PSD files in Poser. Switching back to win98 gained the PSD but lost gif support. Odd but true.