My whole PC keeps freezing?

My mouse, keyboard and whole system keeps freezing. Sometimes can run all day fine, but other times like this morning, it has froze 3 times within half hour. When this happens, the clock freezes so the only way I can reset is by turning off at the wall, as ctrl alt & delete does not bring up the task manager. I have tried all sorts of programs ie: advanced windows care, ccleaner, antispyware etc but not working. I have also hoovered out all the inside of the box, but still not working. I am after some new ideas if possible, before the only thing is to buy a new one....... (does not freeze if we are working on it, only when nobody is using it, and I have checked the power settings in the control panel which 'never' turn off any power supply.