XP isn't supposed to have the same memory problems as 95 and 98...but IME, with Poser 5, it does. I have XP, and I have to save my work and shut down Poser every once in awhile, or it gets ridiculously slow, or even locks up. Sometimes I have to reboot. After that, it's fine. Rendering in particular seems to cause some kind of memory problem if you do it too much without shutting down Poser once in awhile.