Help - Cannot open HD

Just got my first Mac several weeks ago. Yesterday I tried clicking on the Mac HD icon on the desktop and it did not open.

I can get into the HD via Go, and then using the side bar to navigate, but cannot open any folders by clicking on them, any ideas?

Applications and teh dock work ok, but I cannot open pdf, jpg etc by clicking on them either.

I have tried resetting permissions, clearing the PRAM and trashed the prefs but still nothing.