When you upgraded to Snow Leopard, how did you go about doing it? Did you just pop the disc in and let it do its thing, or did you do a clean install?
If the former, you may want to make a full backup to an external using something like SuperDuper. Then, boot off the Snow Leopard disc, erase your drive using Disk Utility from the Utilities menu. Then, do a clean install. After the install completes, you'll be given the option to use Migration Utility to transfer files from "another volume on this Mac". At that point, connect the external drive and do just that. That will bring in all of your old documents/data/applications.