Osx Snow Leopard won't boot up

Hi,
I'm having trouble with an original Osx Snow Leopard on my Mac Pro.
For some reason my Mac Pro won't either update from the original DVD nor will it erase and install when I choose utilities.
I tried various ways including pressing c from start,going to start up disk and choosing the install DVD as the start up disk...I even tried installing a fresh copy on a separate hard drive and still won't work.
Any suggestions please ?

Thanks,
Carlos Curmi.

G'day and welcome to the forums.

A little confusing. You do not update from the install DVD. You update using Software Update in the Menu Bar under the Apple icon. And tell us about the Snow Leopard disc. Is it a full retail install version or a silver grey disc from another computer which will NEVER install?

After that Carlos sounds like you have a dodgy optical drive in your Mac Pro and it is a Mac Pro and not a MacBook Pro?

OK. When I said update I meant that Snow leopard has two ways of installing itself .One where you can choose to erase and install and one that just re-installs the Os without erasing nothing.
So this morning I tried doing first aid from the disk utilities and when I tried verifying the disk the message came up that the disk needs to be erased and that I should install a fresh copy of OSX.
So then I try the procedure again where I choose utilities from the Osx Snow leopard installer the system restarts but it won't go in erase and install mode, instead the screen freezes with the desktop picture associated with snow leopard...the purple kind of pink with stars in it.......with no icons on the desktop what so ever not even the mouse pointer.
I have two separate hard internal hard drives and an external one ,so now i'm thinking to take out the hard drive with the system on it and try installing a fresh copy of the system on the other hard drive.....I suppose if this doesn't work then it has to be the optical drive although I am still able to play and burn cd's which is strange?
Thanks for the information...please share if you have more....

Carlos.

By the way yes Snow leopard is a full retail version and the grey ones that came with my Mac Pro do the same thing they starrt but don't run.....the screen just freezes....
Will keep you posted on the removing the hard drive theory...
Thanks again,
Carlos.

I have just also ordered snow leopard software off apple website i received it this morning but as i bought a secondhand macbook it will not work either its like it wont read the snow leopard cd then it just ejects itself constantly but normal computer cds work as i have tried doing it the external hardrive way but i gave up and bought snow leopard, my macbook i bought is the 2007 version with tiger software on it as i want itunes and other software i cannot because it keeps saying i have to have software 10.5 or higher.. i spoke to a mac tech on the phone he said its probably the cd dvd drive but i dont see how when it kept the windows cd in it for hours without ejecting.. in my opinion apple has made me waste my money because now i either gotta pay for someone to fix the cd drive or drive an hour or more for the mac tech i spoke to to install it by hardrive way for free..

if you could help me i would be very happy to get it over and done with