An Archive and Install does that:
How to Archive & Install:
and this also:
BUT: Don't install older versions of Mac OS than what came with your computer:
There is no Archive and Install option in 10.6, Snow Leopard. Using the Install DVD, if you simply choose Install, the default will be to save all third party applications and user data. However, while not likely, things may still go wrong. I wouldn't do this without a reliable backup, a bootable clone or Time Machine.
When finished, you will have system files as of the the version on the install DVD, most likely either 10.6.2 or 10.6.3. Then update using the 10.6.8 Combo Update from Apple downloads and run whatever is needed from Software Update.