Windows XP will not restart

I came to this forum for an answer and I did somewhere. I had the same problem, my Windows XP would not start up, I would get a black screen, so I started again and when it got to HP screen I F11 and got menu to start normally, start at last known good configuration or start in safe mode. I chose start at last known good configuration and it started. I was so happy. I had downloaded AVG] internet security and during that time it went down and would not start again, somehow it is not compatable with Norton Security Suite also. I had received a pop-up pop up download AVG to clean computer and I will not do that again, I'm going out to but extra protection, forget getting it online, TOO RISKY!