If you installed Gnome Classic desktop in Ubuntu 12.04 or 12.10, you may have already found that the Alt-Tab windows switching not working any more. So this simple and brief tutorial will show you how to enable Alt-Tab windows switching behavior in Gnome Classic.
First go to Applications -> Ubuntu Software Center, search for and install the CompizConfig Settings Manager. Then launch it under Applications -> System Tools -> Preferences.
In the left of CCSM, click on Window Management, then check the checkbox of Static Application Switcher at right.
That’s it. Enjoy!