My computer keeps freezing.what do i do?PLEASE PLEASE PLEASE HELP!!!?

I hear that and immediately finger your model as one that had a plague of issues with faulty power supplies and bad capacitors on the motherboard. The iMac G5s were lousy with them. If you don't mind, unplug it, put it face down, unscrew the three screws on the bottom (they don't come out, they just spin for a while and stop), and pop the back off. Look for bulging and/or leaking capacitors. If you see any then you'll know that it's a lost cause and a new computer is the only viable solution.