I need instructions on how to bypass sad mac page
It's not a virus, it's indicating an error either with your hardware or the Operating System. What to do depends upon what Operating System you are running, but it's probably time to reinstall it.
I second ijack's reply but I would add that if it's a Sad Mac icon, it's probably more likely hardware failure. You might try ijack's suggestion and try reinstalling the operating system, but if that does not work you might try taking it to an Apple Authorized repair area (you can find locations in your area on Apple's website). Only thing I'd be worried about is, if it's a G4 like you say it's most likely not under warranty....so be prepared to pay up for any repairs.
Best of luck!