When you say you allow these authenticated users to preview the course, are you wanting them to see everything in the course, or just a few preview items?
You have a few options if you play with groupings. If you group your assignments (maybe make an 'enrolled' student group), then you can prevent your authenticated users from seeing those activities at all, which also means they cannot mark them as complete. You could group your lessons as well and maybe leave just a few ungrouped (or turn off the activity completion for those few preview lessons) so your authenticated users can view those as preview lessons and wont have the ability to 'check' them as complete.
Other things you can do is group the actual topics so you can avoid grouping individual assignments, which I find to be a nice time saver. You could even make a topic that has a few preview items in it just for those authenticated users (so when they see the course, they only see those preview items if the other course items are grouped). The trick is then to have your actual enrolled students not see those preview items. Moodle 2.8 has a few more conditional settings that can aid in this later item (or you could go into the permission settings for those preview items and remove the 'student' role so your enrolled students wont see these items). It's almost like having 1 class that does 2 things -- serves as a preview for your authenticated users, and serves as a class for your enrolled users.
I hope any of that makes sense to you. I have a CyberLibrary that I have to do these settings to so various users who enter see different things. Keep us posted on how this works out for you. Moodle is so powerful, I'm sure there are other ways to accomplish what it is you are looking to do that I'm not thinking of/aware of.