So you finish installing your Ubuntu and playing with the function buy hey the ALT-TAB is not working!! Here’s a workaround.
So how you can enable ALT-TAB in GNOME Classic in Ubuntu 12.04 LTS (also work with 12.10)
If you have GNOME installed skip to step 2
STEP 1 : Installing GNOME Classic:
- On the terminal window (CTR-ALT-T) execute this command :
sudo apt-get install gnome-shell
- Once it finish installing Log out and choose GNOME Classic on the login screen.
STEP 2 : Install and use CCSM (CompizConfig):
- On the terminal window. Execute this :
sudo apt-get install compizconfig-settings-manager compiz-plugins
- Once it finish setting up still at the terminal execute CCSM by executing this command :
ccsm

STEP 3 : Click Window Management in the left panel.
- In the right side, Click the checkbox next to Application Switcher.
- That’s it. ALT-TAB should now work for you in GNOME Classic. ENJOY!