This 99% of the time means a bad logic board, unless you're very proficient, have the time, and are willing to spend the money, it's not exactly worth it to replace the board. What you can do, however, is salvage your data by taking out the hard drive using [guide|703|this guide] and putting it in an [http://www.ifixit.com/Apple-Parts/Firewire-ATA-Hard-Drive-Enclosure/IF107-072|enclosure], from which you can transfer your data after replacing your computer.