See method Kappy described, to perform the task without using the incorrect
install-restore DVD, (since you can't boot from a 10.5 install DVD to use that
version of Disk Utility, to repair an existing 10.6.8 system) the prescribed path
says to use Single user mode; that original link (in blue, in that post) fails now.
For instructions to perform access as suggested in above answer, see this link:
•Mac OS X: How to start up in single-user or verbose mode - Apple Support
To see how that may play into the overall picture, if you have no command-line
or single-user mode experience, you may have a learning curve. See basics:
http://www.tekrevue.com/tip/mac-startup-options/
Troubleshooting startup issues, resolving some, or see Specialist/genius.
•Mac OS X: Gray screen appears during startup - Apple Support
Or you could contact Apple sales online or main site, to purchase new retail
Snow Leopard 10.6 DVD; you'd need to tell the operating system specialist
your computer's serial number to be sure it would be a correct match. ($20)
http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard
Good luck & happy computing! 🙂