It would have to be XP, on the grounds that that's going to be supported for longer. XP Professional is a better bet than XP Home, if you have that choice. I use 2000 personally, and haven't felt the need to change. Poser isn't very demanding on the OS, except that NT based versions of Windows are superior to 95 / 98 / ME, mainly because of their improved memory management. One thing that afflicts Poser 4, and I don't know if it applies to 5 and 6, is sensitivity to page file fragmentation. A good strategy, if you know your way around OS setups, is to create a small partition on your hard disk (say 2-3GB) and use that for the page file only.