The iMac doesn't recognize the Macintosh HD, what can I do/replace?

Three months ago, I bought Mac OS X Snow Leopard, and since that time, each time I turned the computer on, a sparkling question mark appeared.

As it is said in the manual, I re-installed the 10.6 version three times, but I has not changed anything.

Yesterday, I forgot to remove the install CD from the computer and when I decided to turn it on today, the installation program worked.

The problem is that I can remove the CD and, even if I decide to install the version one more time, the Mac doesn't find the disk (Macintosh HD).

I followed the instruction even the ones that uses Terminal, but in the column "Mounted on" there was only "/", contrary to the other disks.

I hope that someone will understand what I'm talking about (I'm french and English is a really vague memory from school) and will help me.