Try trashing (but don't empty the Trash just yet) any com.applesystempreferences.plist files you find in this location. It's in your Home Folder Library>Preferences>ByHost
Also look for any in that location, but not in ByHost and trash them. You should have this one as well.
Does System Preferences open from another account? Create a new admin account and test for this there.
Also, no need to type in a large bolded font.
Please post you iMac version and the Mac OS it's running. iMacs have been in continuous production since 1998 in four distinct families, so it helps to know which one so you don't get inappropriate advice for your model or operating system.
"About this Mac" from your Apple menu with show the processor description--which will tell us which iMac family you have---and your Mac OS version.iMac 27-in; Mid 2010 2.93G i7 Quad, Mac OS X (10.6.8), 4GB RAM; ATI5750 1G VRAM