To reset Unity to all the defaults:

unity –reset

Compiz holds its own configuration, you could try re-configuring compiz:

dpgk-reconfigure compiz

You can easily reset Unity or Compiz using the following commands
(be careful when using these commands and only use them if you really have to!):

• to reset the Unity launcher icons:

  unity --reset-icons 

• to reset Unity:

  unity --reset 

• to reset Compiz:

  gconftool-2 --recursive-unset /apps/compiz-1 unity --reset

  If you are using Ubuntu 11.04 Natty Narwhal Alpha, you may face compiz crashes. To recover from compiz crash, you can do a

  compiz --replace

  If it does not work, login to the terminal (Ctrl+Alt+F1-F6) and remove the compiz settings directory from your home directory. Here is how you do that:

  cd mv .compiz-1 compiz-old

  Now, restart compiz:

  DISPLAY=:0.0 compiz --replace

  sudo apt-get install ubuntu-desktop 
  Replace ubuntu-desktop with the desktop you wish to have 
  
  ubuntu-desktop -> Ubuntu (Gnome/Unity) 
  kubuntu-desktop -> Kubuntu (KDE) 
  xubuntu-desktop -> Xubuntu (XFCE) 
  lubuntu-desktop -> Lubuntu (LXDE)