Can I repair my system disk (10.6) by booting off a 10.5 install CD?
Can you boot into Snow Leopard at all?  If you can, try this command from terminal: 

sudo diskutil repairPermissions /

Broken down this does the following:


sudo - run this command as superuser/admin
diskutil - run the diskutil program
repairPermissions - obviously this means to repair the permissions
/ - act on the whole file system


If you can't get into Snow Leopard, you might be able to get into single-user mode by holding &#x2318;+S on boot

You can run Disk Utility from within Snow Leopard but whether it can repair it will depend on what's wrong.

You can do a Repair Permissions on any disk, or a Repair Disk on any disk which is not your startup disk. If you need to do a Repair Disk, you will need to do that from your CD and I think the 10.5 disk would be OK to use for a Repair Disk when booted from, it may not do a Repair Permissions though.

It's worth trying the Repair Permissions first from Snow Leopard. That solves a lot of issues