This problem first started last year: as soon as the power plug was switched on the system powered itself up then shut down, then restarted then shut down etc. At first I thought it was an on/off button problem and one of the electricians at my work swapped the main button for the reset switch and this seemed to work. Last week the problem started again and I texted a computer engineer I know and he thought the CPU may be over-heating, so I got the sides off the tower and vacuumed and even got a paint-brush into the fans (and a load of dust came out) and at first that had no effect, but about 2 days later the system worked fine (apart from one shutting itself down episode) Anyway (bear with me) yesterday the same problem repeated itself: turn the power on at the mains, system starts up, shuts down etc. No error messages have ever shown. So I got the sides off again and turned a large fan onto it for about 2 hours but still no joy. So I took the system to work with me and put 4 stand up fans around it for 3-4 hours and then the PC started to work - it took quite a few 'self fire-ups' which sometimes meant it got to the Windows log on screen then shut itself down etc. Finally it fired up and worked for 5 hours until I turned it off, I got it home and it worked for another 3 hours then it shut-itself down (didn't crash as such) and spent 15 mins trying to re-start until it finally managed it. In summary I don't think it's a Virus as my protection is pretty good and up to date, plus it never even gets loading Windows when it's doing this. If the CPU is overheating why has the system come back to life barely 15 minutes after shutting down? and is still running 20 mins after that? Is the problem (as the PC engineer mused to himself when I spoke to him) a problem with the Hard Drive loading? Could it be as simple as the on/off button? (Even though the wiring to and from it is good and not broken) Alternatively, the system is about 5 years old and I was planning to upgrade it this year, so should I just do that and put all the bother down to hardware getting old and dusty? Sorry if this is a rambling post, but it's unpredicatibilty is really pissing me off?