I was recently working on a G5 that my friend had been having some problems with... heres what i know about it
It was running and she left her house - she came back to a very loud fan running near the front of the case (i'm assuming this is the CPU fan) and more importantly the monitor was black and unresponsive. When she boots the computer now during the splash with a the grey apple logo on it and the spinning grey circle underneath it, the grey circle is surrounded by white for about 2 or 3 spins. the grey spinning circle is then in a small box or red distortion. the computer proceeds to boot up to a screen with crooked bars across it and is unledgable.
my initial thoughts are that it may be the graphics card, but the last time i have worked with OSX it was on my computer with a cracked version of 10.4.5 so i'm no mac expert. Which leads me into my next question, how do you get to a boto list so that one can boot into a "safe mode" like scenario?
I have a G4 and when that happened to me the screen soon proceeded to go blank and I realized the LCD had burnt out.
Holding down shift will boot up your MAC in safe mode. If you hold it down from startup until after you are logged in it turns off some kernel extensions, items you have set to open at startup, and nonessential USB devices. That's the only way I know of, aside from trying to run it with the startup disk.
Except that they describe the shift key as 'closing open windows' and I think that 'safe-mode' would have been a better description.
Here's another article warning about strange behavior attempting safe-mode with a CD left in the drive. I've seen a lot of weirdness over the years booting macs with a disc left in the drive. So mouse button may be a good one to try first--see if there's anything in there.