Infinit loop/Direct3D error

I get alot of blue screen crashes and game crashes (to the desktop) when playing the sims 2 with all expansions, though I far exceed the system requirements. When it crashes to the desktop, it says :

Direct3D returned an error: D3DERR_NOT AVAILABLE!

When the computer crashes to the blue screen, it says that something was caught in an infinit loop and it had something to do with nv4_disp (sorry, I copied down the entire message, but misplaced it!).

Anyways, I have tried uninstalling my graphics driver and reinstalling it, but when I install a new driver, there are light horizontal lines going across the top of the monitor (the lines appear to be a distortion of the image at the top of the screen). The only way to get rid of this problem, is when I install a SECOND, DIFFERENT driver over the first newly installed one, without uninstalling the other one again (which I know I am not supposed to do). I currently have the Nvidia Geforce 7600GS at 512 MB memory, as well as an integrated graphics chip, the Nvidia Geforce 6150 Le. I assumed the integrated chip would automatically disable when I installed my new card (which I believe it has, as the new card is the only one listed in the control pannel), but could it be causing interfere? And back to my original question, could my "new" graphics card be the problem, and is there a FREE way for me to test it and find out?

I'm sorry if my questions come across as stupid, or if they make no sense! I have a very limited knowledge of computers, and didn't know where else to ask!