Do you mean NVidia 9800?  Most of the time an error like that is video driver or hardware related.  Other causes could be memory (not enough or having hardware problems), A Virus (Unlikely if you are using up to date antivirus software.)  Or even other software running in the background which is messing up memory or video subsystems.

The think about Carrara is that it uses a HUGE amount of system resources compared to most other software you probably have on your system, including Poser, DS and Flash.  It is also very touchy about video drivers.  I actually had to drop back a couple of versions when I first got C 6 Pro.  The newer video drivers were not working correctly with it.  I am also assuming you have the latest DirectX nd Open GL software.  Try making sure you are not running ANY software which the system or Carrara doesn't need to be running before you start Carrara.  Try and see if there is a new driver out for your card.  See if these help. 

I have had some pretty serious problems with 7 myself, but not exactly the same as what you describe.  My problems all happen in the shader room, then the whole program goes unstable.