Amazing I think leaving the computer on will not hurt it. Many people claim it will cause an accumulation of dust and dirt to appear on your Motherboard and fans causing premature breakdown. But after seeing my share of computers I know you're going to have dust and dirt accumulation whether it is left on OR off. I am sure you are not using Windows 9x, which needs to be restarted from time to time for optimum performance. As far as cleaning goes you will need to clean your machine every six months or so to keep the dust and dirt from getting too thick on your chips. And if you smoke, clean your machine a little sooner as the nicotine from tobacco smoke will stick to your chips and attract more dust. If left too long it will actually cement the dirt and dust to the chip severely shortening the life of the chip as it cannot diisapate the heat as fast as it would if it were clean. So yes leave it on but make sure you clean it regularly