Can not reboot from original OS10.6 DVD

I'm Running OS 10.6.8 and trying to do a disk repair on my Macbook pro. When booting with the original Mac OS 10.6 (Snow Leopard) DVD it will go only to the white apple on the light grey screen.

I also have a bootable system hard drive with OS 10.6.4. I tried to boot on that drive my Powerbook and then my iMac and with this one it would hang on the blue screen with the pointer on.... I would like at least to install the latest upgrades on that bootable (emergency) disk

Is there an incompatibility to backstart with the older disk once you upgrade to 10.6.8?

Please help, thanks

You have mentioned 3 different machines, so we're all a little confused.

If you are using a gray disc that shipped with one of your Macs, it will not install on any machine except the one it shipped with.

If you are using the correct disc on the MBP, clean the disc and also try 2-3 quick bursts in the DVD drive slot with a can of air (while your machine is shut down).

You will not be able to boot 10.6 on a PowerBook. 10.6 is Intel only and will not run on a PPC Mac.

Thanks. It is not a restore disk, it is the original System DVD disk purchased from Apple when I upgraded to Snow Leopard.

It's 2 machines not 3, sorry for the confusion, did not mean PowerBook, but MacBook Pro: I have an iMac 2.8 GHz and a 17" MacBook Pro, both intel, both running the same OS X 10.6.8.

Tried cleaning the disk, it is pristine. The DVD drive works well with any other disk.
And then it is the mystery with the Snow Leopard system bootable hard drive that hangs too, on the blue screen with pointer, same on both machines...

Thanks for your replies !

Best

Vlad