If your iMac has been infected by a virus, is running slowly, or having other system issues, a system restore will restore your iMac to the factory settings, removing any system problems. No matter if you are a beginner or advanced iMac user, completing a system restore is a task that anyone can do by using the Mac OS or Mac OS X installation CD. Always remember to save any important documents, folders, or media files to a CD or DVD before completing a system restore.
Turn on your iMac.
Insert the Mac OS or Mac OS X installation CD into the CD/DVD-ROM drive of your iMac.
Press the "C" button as your iMac begins to boot up, ensuring that your computer will boot from the installation CD.
Click on the "Utilities" menu when a new window pops up, and then click on "Disk Utility."
Select "Macintosh HD" and then click on the "Erase" tab. A dialoge box will pop up. Once this happens click "Erase". This will erase your hard drive.
Click on "Quit Disk Utility" once the partition is erased.
Click "Continue" when the next window pops up after you have closed "Disk Utility". This will open up the OS or OS X Leopard Installer screen.
Click "Agree" when the license agreement pops up.
Click the green highlighted arrow in the window that pops up. This highlighted arrow is pointing to the disk you just erased.
Click "Install". Mac OS or OS X will now begin to reinstall. There will be status bar at the bottom of the screen, indicating the progress of the installation.
Click "Continue" once the installation has completed. Click "Restart" once the "Installation Successful" screen pops up. You have now performed a system restore on your iMac.