Clearing the PMU/SMU and resetting the PRAM worked only once on my iMac G5. I shut down the computer at the day's end. Upon booting up again, the white page with a dark gray folder with a question mark in its middle flitters I'm the middle of the page. After several additional tries at resetting the PMU/PRA as well as PRAM routine the question mark folder is still flashing. In the final analysis, Apple Care recommends reinstalling the original OSX install disks. Hopefully, you will have everything backed up since reinstallation may damage/delete some or all of your files.