Video drivers are software. Your ATI video card works fine. People running Windows have problems. Your video drivers are written by XFree86 contributors. Windows users' video drivers are written by ATI.

Draw your own conclusions.

Personally, I think that ATI software deveoplers spend way too much time trying to make their software avoid Windows UI specifications. I've always had problems with ATI software, both technically and aesthetically. I've occasionally had technical problems with nVidia drivers, but less often than with ATI. I've never had any aesthetic problems with them. So until ATI gets its software act together, I'll continue to avoid them, at least for my Windows machines. I'd prefer to avoid them elsewhere as well, in hopes that the lack of sales will encourage them to write better drivers for those stuck with crappy OSes.