Guest account on MAC OS X 10.5 can go to Finder and see all the files
The most likely culprit is some utility or old step that was taken to change the permissions on your folders. Depending on how this happened, you may have a little or a lot of clean up, but let's verify this is what is happening:

Here are two screenshots of the default permissions for your administrator's home folder as well as the Documents folder within the home folder.





You can edit things by clicking the lock icon, entering an admin user/password and then using the +, - and gear buttons. Don't forget to apply the changes to enclosing folders. Also, getting a little help from Apple or a tech is often needed if many folders have incorrect permissions. The ~/Library and ~/Shared folders have permissions different than most as well as files that are constantly in use so try to stick with Photos or Documents until you're sure you're fixing things appropriately.