Very Strange Error HELP

My mac has been acting very strange lately. This morning the computer had no dock and was operating very slowly, I was constantly getting the pinwheel. Now something stranger has happened, the dock has returned but now I have an even worse problem. Now all of the writing on the computer seems to be A's with small boxes around them. None of the applications will respond and I am constantly getting the spinning color wheel. Restarting the computer does nothing, please help!