24" MAC freezes on start up with no entry sign-II

Right, this is how is all started:

• I turned on my MAC and got the gong, grey screen, spinning wheel, and apple.
• I then got, instead of the apple, the no entry sign after a longish wait
• I started in safe mode and repaired permissions.
• Repaired he HD too (no problem, according to the disk utility there.

No progress.

So:

Having bought my iMAC with Leopard and later upgraded to SL, I started with the Leopard start up disk (installation) that I got with my computer, entered in disk utikities and erased the disk.

I then re-installed Leopard.

I then re updated SL

I then restarted with Leopard again in order to enter disk utilities and access the Restore from Time Machine option.

I went through the process of restoring from TM

When the process was supposedly finished, I was asked to click on a button to re start, which I did.

The Surprise:

The computer then started up, gong, grey screen and..... a line scanning the screen from top to bottom darkening the screen a little and then a dialogue box appearing (black box, white text in several languages), telling me that "..I needed to restart the computer, I should press and hold the power button till it shut down and then press it again to restart it."

I did. It didn't work, the same instructions to press and hold the power button, shut down, release and press again appeared again.

Now I shut down the computer pressing and holding the power button and feel desperate. (I am writing from my MacBook Pro)

Where have I gone wrong?

Please.... Help!