I have a 24" iMac with Snow Leopard. Before this problem occurred, I hadn't noticed anything wrong with it. I shut it down, and the next day when it auto turned on, I logged in and got visual glitches. Random groups of pixels. I reset to try to get rid of them, and it never booted up all the way. I reset the PRAM and NVRAM and the groups of pixels changed to what look like scrolling lines of pixels, matrix code-like. A friend of mine had fixed it, for a bit, then they just appeared in the middle of me using the computer. Now it's back to not being able to be turned on at all. It gets to the Apple and the spinning gear spins for a while, then stops. But the pixels never stop moving. Sometimes I get past it, but I get other visual glitches, like a black screen with a flashing box on the upper 1/3rd or so of the screen. Or a blue screen with random pixels. My PC friend called them artifacts and said it might be my graphics card. The only problems I've had before are memory problems. Any ideas? I've also tried starting from CD, safe mode, hardware test, and holding down x.