When I log into the Gnome session, the shell does not launch.

In terminal I get the following:

pst007x@pst007x-Serval-Professional:~$ gnome-shell

Window manager warning: Screen 0 on display ":0" already has a window manager; try using the --replace option to replace the current window manager.

pst007x@pst007x-Serval-Professional:~$ 

How can I find the current window manager?

If I change the window manager will it break Unity?

Any suggestion?

Thanks

Ubuntu 13.04 64bit
Gnome 3.8

EDIT:

Removed Compiz and Unity, then Gnome-shell worked... Guess it's a conflict somewhere