Poser has know memory bugs that are independent of the OS (and supposed to be patched soon), so I'm not sure if one OS is going to be better for it over the other in this regard. I personally like XP Pro better; Win2K is not nearly as user friendly and is not as well supported by software makers and device makers. Given that MicroShaft has a long history of dropping support for older products, I think you would be better off with XP.