iMac graphics problem

Hi to all! My iMac has failed and I suspect it's a problem of its video card. What happens to my iMac when I turn it on:

1) A grey screen, very briefly;
2) The grey screen with Apple logo and spinning wheel for a very long time;
3) A blue screen very briefly;
4) Then, the screen completely corrupted but mouse still moves around with a small pixelated square attached to it.

My iMac specs are:
MC413 J/A
iMac 21.5
3.06 GHZ CORE 2 DUO
4G GA30059

I hope someone can help me here in this forum.

Thanks in advance.
Rosalina

One way to find out if the graphics are defective is to attach another monitor to your iMac to see if it displays the same thing. You will need an adapter and proper cable in order to do that. You can also use a TV set if you do not have a spare monitor.