setting permission

I would like unregistered visitors to be able to view all the content on my front page, which includes excerpts of the latest posts, pages, and forum topics. However, if a visitor tries to view an entire post/page or go anywhere else on the site I would like them to get an error telling them to sign up if they want to view the page.

Is this possible? I know I can disallow 'permissions to view content' in the administer section, but this removes all the content from my front page until I sign in!

Any help is much appreciated.