Why do I keep getting a blue screen on my computer?

A few weeks ago my screen began to seize up, glitch, go black, come back and then give me the message that my display driver had stopped responding. It would do this several times a day.
A few days later my computer began to turn black, try to come back, then I would get a blue screen with all kinds of messages on it then go to black until I got a message asking me about how to I wanted to restart windows. The first time I restarted in safe mode, scanned for malware, found none, rebooted into normal start. Now I just go ahead with the normal restart when this happens.
It used to happen only after it had sat unused for a while so I thought that it perhaps was a problem with the sleep mode. I turned it off but it did not solve the problem.
It has gotten worse over time. Yesterday it crashed about eight times, most of the time while in use. In addition, I still have the problem with my display driver not responding. Today I am not halfway through the day and it has gone down three more times. Also, windows will stop at odd times and I will have to sit and wait for it to “catch up” for lack of a better term before I can proceed with whatever I was doing.
As I said, I have checked for viruses and malware using three different programs.
The system is a Dell OptiPlex 330 using Windows 7.
Any ideas?