Unable to install snow leopard (circle with diagonal)

Hey all.

I have desperately tried to install snow leopard on my mac, but I haven't been able to yet. I have the installation disc, but my cd drive doesn't register it. I have then borrowed an external drive for mac from a friend. I have tried all the tips in the installation guide for snow leopard by apple, but nothing have worked.
The thing is, it seems like the computer can register the disc, because when i go to start-up screen while holding option I can see the disc as an option (besides my 2 partitions). But it won't register it while I'm at my desktop.

I have tried getting it to start from the disc by selecting it after holding option while resetting, but after it runs for a while (i can hear the drive running) showing the mac icon, it starts showing the attached signal, a circle with a diagonal line.

Can any of you tell what seems to be the matter?

I live in Denmark, and my warranty have expired long ago (bought the mac in early 2008). I think it doesn't seem to be the drive but something with the software, since it is able to recognise the disc in the start-up menu but not on the desktop.

Thanks in advance.

Sincerely


specs:
Mac OS X 10.5.8
Model Name: MacBook Pro
Model Identifier: MacBookPro4,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.4 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 3 MB
Memory: 4 GB
Bus Speed: 800 MHz
Boot ROM Version: MBP41.00C1.B03
SMC Version (system): 1.27f3
Sudden Motion Sensor:
State: Enabled

 

I have bought the retail version, sorry if I didn't make that clear

Sincerely.

 

Hi guys/girls,

thanks for all your help. At first I thought it might have been a problem that my computer is bought in the US, but since I live in Denmark, I bought the danish retail version (it is probably the same around the world). But then I tried using the external drive to load another CD (computer game), it didn't show up either. And since I can get the computer to recognise the install disc (SL) on the start-up screen, I reckon it's a problem with the software? Somehow it isn't able to load a cd while OS is running, by which I of course mean after start-up screen, where I see my desktop and use internet, programs etc.
Anyway to fix the software issues? Any ideas or am I missing something. I will of course try and use the method mentioned by Makosuke, but it seems like a more serious problem, since it doesn't recognise any cd's and it doesn't seem to be the drive, but the software.

Thanks again for helping

Btw my cd looks the exact same as the one on Quad5ny's picture.