OK I've been going crazy here because I thought this worked on my other computer but it doesn't on my laptop with snow leopard. A little more searching and my friend google says that Mollie is right this doesn't work in snow leopard. However I did find another link that explains how to get it to work but you have to go into the terminal.
Mac 101 | Tips and Tricks: Enable Folder Previews in Snow Leopard
It might be work calling applecare and getting them to walk you through this. My other thought is that apple is having a big announcement on Jan 26...which typically means upgrades on things that may have been causing problems.
Sorry that it hasn't been clear and I hope I haven't added to the confusion.