Comment 5 for bug 876999

OK as a temporary workaround:
Step 1: Login to Unity
Step 2: type Alt-F2 (or fn-alt-F2 on a Mac)
Step 3: type "killall nautilus" (without the ") and press enter (you will now be able to see windows, but don't open any nautilus windows)
Step 4: install pcmanfm (or other file manager either from terminal: "sudo apt-get install pcmanfm" or from Software Centre)
Step 5: open a window with the new file manager and add it to the unity launcher
Step 6: remove the nautilus home folder from the launcher

When you reboot and log in again, you would have to follow steps 2 again, so to prevent that there should be a way to automate the killall nautilus command. However, I tried to just put it in the start-up applications and this killed the gnome session.