XP Pro is the way to go. Used Poser 5 for a while but I have noticed it is heavier on system resources than Pro Pack, which I prefer and currently use with Lightwave 7.5 Using VAIO laptop, P3 1gHz, 512mb RAM, 40gb HDD to create scenes, using 2 machines to render; both are Athlon 2.1gHz, one has 512mb RAM and one 120gb HDD the other has 1gb RAM and two 120gb HDD.