My computer keeps frezzing! :(?

My PC will rarely be turned off properly because it frezzes so much. I have cleaned out the registers many times and the problem still persists. I dont think that it runs low on RAM as it still does it when nothing is running (and i have a thing on the system tray telling me my free RAM and it never hits 0) I have optimised the graphics for performance apparently and i dont know what to try next. Any advice?