I use PPro on my older Opteron 185 Windows XP x64 box and on my newer C2D Vista x64 machines and it works fine on both.

If anything, on XP x64 it seems like it renders a little faster (Vista is pretty bloated), but Vista is more stable. The one killer thing I run into on XP is that PPro does not like to have multiple monitors enabled, it majorly slows done the OpenGL preview, Vista does not have this issue. Since I work on dual 22" screens, I perfer the Vista setup.