2000 Pontiac Firebird Security light, Car wont turn over

Hi,
Just recently I went to start my firebird and it would not turn over. I put my key in and everything works fine, theres lots of power there but it will not turn over and the security light comes on. I did some research and found out that it could be my Pass Key so I cleaned it and retryed starting the vehicle but the same thing happened. A couple times when i've been driving the security light has come on but it has gone off after about 5 min of driving. Also just recently I have installed a aftermarket stereo deck do you think this may have anything to do with it? I dont know if it is just coincidence but the security light never came on before I put the new stereo deck in. My key appears to be in good condition so I don't believe that is the problem. Any ideas? I hear that sometimes repairs can be costly with this type of problem, is there a way I can override this safety feature and save a few bucks?