I've yet to experience a single crash with Poser 8 (or Poser Pro - no crashes since P7 actually), so I wouldn't go laying the blame on Poser or Vista just yet. There are all kinds of software and hardware configurations that can cause problems. The hard drive thrashing doesn't sound like it's a problem with Poser, but a systemic issue instead. I'd be interested to see how much of your system resources are being used by Poser while rendering a scene that gives you trouble. If you're using a bunch of figures and props with high resolution textures, 2GB is not hard to fill. Keep in mind that textures are uncompressed when being rendered, so a 4000x4000 texture that only takes up ~1.2MB on your hard drive requires 45.8MB of memory uncompressed.

That said, I doubt it's your display drivers either, especially when it's the rendering engine causing problems and not the Preview window. Assuming you're using the Firefly engine to render scenes and not OpenGL, your GPU is not utilized while rendering at all.