Pull out the DVD's that came with the Mac new, and put the restore disk in. Then turn it on and hold down the "C" key. It should start with the disk instead of the hard drive. If you still get the red circle, you have a hardware issue, and need to take it to Apple to get it serviced. If it does boot, then select the language you use, then a Utilities menu will show up at the top. Click on it and select Disk Utility. The app will launch. Select your hard disk in the left pane, and then in the right pane select first aid. Have it repair the disk. If it finds anything, it will try to fix it. If it does, run it again, and keep running it until it makes no repairs, then repair permissions. Also run that until it no longer makes any repairs. Now you should be able to quit that app, then quit the installer and the Mac will reboot. If you still get the red circle, the only other thing I would try is reformatting the hard drive and reinstall the OS. Only do this if you have your files backed up on a different disk. Otherwise it's a trip to Apple.