Install Previous (newer) Version of OS

For reasons I will not go into (as they are slightly stupid). I installed OS 10.4.7 from disc onto my imac that had recently had Snow Leopard installed on it. I was prompted to save the old OS as a previous install and I can see all the folders on my Hard drive. I want to revert to Snow Leopard. Is it possible to do this?

backup your system onto an external HD, install SL from the disc, and restore from the backup when you are done.

Sorry. I should have said in my first post that the SL install was done by Apple store when they put a new hard drive in for me. Last one failed. I do not have the discs, just the files on my HD.