Hi, Vista 64 Ultimate here and Poser 7 and even 6 run fine, The only problems since day one have been related to the nVidia drivers using openGl mode  which in Poser only affect the preview screen or render anyway. For a while before sr3 openGl was also working in preview mode without hassles, but since sr3, SreeD is way better being very stable.  With enough ram Poser preview and renders are faster than XP Pro on my machines anyway. Another horrible culprit can be the anti-virus you use. Windows new Live version actually runs fine as does TrendMicro. At one point I tried CA because it runs on Vista, was on Microsoft's certified Vista list and is the choice of Fortune 500 companies. While that company claimed no perceptual slowdown of the system with use, I found it to be just the opposite, It's safety features were more cumbersome than Vista's AC. and even surfing on a fast connection was slower than a regular modem due it's constant preview checking.   Hope this helps.
Regards...