OS??? It's windows xp. I took it to a computer store and the guy played with it a bit. He changed the power supply and it started to work. Then, when I got it home, pretty much the same stuff was happening, only this time with a little more 'umph'. The fan on the inside will completely spin now and the lights stay on inside as well as out. However, it seems as though the hard drive isn't working because the monitor is in a self test mode the entire time, as if the computer were off, even though everything is lit up. Normally, you turn on the cpu and the monitor will start up as if everything were normal. It's not receiving the signal from the drive saying that the thing is on. I have tried unplugging the whole thing, leaving it for hours, trying it again, and still I get nothing. It simply makes me think that something isn't plugged in correctly and isn't able to give the entire system juice or something.