Hey Joanne !
First, I'd try to upgrade the graphics driver. Just search for your specific graphics card model on the nvidia homepage.
Secondly, if you still get crashes, search your harddrive for a file calles somewhat 'hs_err'. If you find one of those files which got created just in the minute of the crash, post it here. But try first to upgrade to the very latest driver.
Let me know ..
m.