help to factory reset iMac 2009

Back up your data using Time Machine. Insert your original gray Mac OS X install disc and restart while holding the C key. Choose your language and click next. At the next screen go to the menu bar on top of the screen and click Utilities, choose Disk Utility. Select your hard drive in the panel on the left and click erase.

Quit Disk Utility and choose to install OS X.

What to do before selling or giving away your Mac - Apple Support

You can try this:

https://discussions.apple.com/thread/2217185?tstart=0

Steps to cure:

1) Shutdown iMac

2) Unplug power cord from back of iMac

3) Press and hold power button for 20 seconds

4) Restore power cord; power on iMac

5) Immediately on hearing Startup Chime, press and hold Command-Option-P-R keys (⌘⌥ P R)

6) Wait until you hear an additional four (4) chimes; the fourth (fifth) chime will (should) be louder and clearer

7) Release the keys and allow iMac to boot normally.

Note: This reboot may take up to two minutes, but subsequent reboots should be less than 30-45 seconds.

Results:

 Boot time 35 seconds

 super snappy app/window behavior (especially Parallels/Windows and Safari/Flash

 New Xbench Score = 286.24 (must admit I thought this would be higher)

 Super Happy owner of the hands-down, most kickass screen on the planet