I am running XP 64 with a P850D and 4gb of ram. Poser runs good, even Flash 8 works fine.
Z-brush 2 was the only app of note that did not make the 64bit grade. I think 3.0 does.

You did not mention your system specs - but as some as said - Vista is hungry for power and Ram...It was designed to make you upgrade to it - hate your machnine - and then buy a new one.
more..More...MOre...MORe...MORE!

I my world, a CAR, after nearly 100 years of development should get 200 miles per gallon, have 3 moving parts and last 50 years. Oh well...