Your machine can't find a system folder. Either your system is corrupted or your hard drive has gone bad. Start up from your system installation DVD by inserting the disk and holding down the "C" key on startup. Go to the second screen pull down menu to Utilities > Disk Utility. See if the drive shows up on the left. If so click on it and run repair till it runs clear. If it does not show up, it has gone south and you will need to replace the drive with a new IDE drive and install a new system.