My 2008 iMac is all pixelated and freezing please help

I am freaking out right now because my iMac just completely freaked out; I was watching a youtube video and then the whole screen went pixelated and rainbow colored and so I shut it down manually and restarted it and then lines went across the screen and it froze on the start up, so I unplugged the wall cord and unplugged it from the wall and let it cool down for more then an hour; and it still does the same thing. I am freaking the **** out. It used to be my dads computer and all I know is that its

~VIN,iMac (24-inch Early 2008)

I dont have anything other then this monitor; no discs that came with it and what not. Please if you know anything; help. Im using my old macbook that overheats and the fans get super loud super quick now and its awful