have you tried clean driver install? btw are you still using driver version 185.93? the latest one is 285.79 for beta driver and 285.39 for WHQL. you can get the latest driver straight from nvidia website. did you overclock your card? unstable clock also leads to card failed to work properly. what is your card model?
Thanks for your help. My graphics card is an INVIDA GeForce 6150 LE. I have downloaded the 285 driver as you suggested. How would I determine if I have over-clocked my card? I have made no changes to card parameters.