Did you update anything before this started?
I run a similar system, Vista Ultimate 64 with 8 Gig RAM and an Nvidia 9800GT card and Poser 7 and 8 run fine.
I don't render with them though, I use Vue 8 Infinite for that.
Any changes to your system may have had an adverse effect on something, do you have a restore point you can try??

Injustice will be avenged.
Cofiwch Dryweryn.