windows management problem

I have ubuntu 11.04, and i use the gnome interface. But now my computer for all applications except google chrome have lost its window border. i have tried metacity -- replace which fixes it for a second then the problem with no border happens again, and after this all my applications except google chrome launch but then freeze, and i when i try to click the show desktop button to get out of it, i get a message it the top left corner saying that " your window manager does not support the show desktop button, or you are not running a window manager. Also work space switcher does not work. But when using the unity interface, everything works fine. How can i fix this?
Thanks in advance for any help provided.