As a heavy user of GNU/Linux I always loved the good old appearance of GNOME desktop environment. When the fancy looking Unity interface started to appear in recent versions of Ubuntu, I had to use the GNOME Classic version and I used it together with Clearlooks-Phenix theme since I loved the good old clearlooks theme too. However, Ubuntu 12.04 LTS is facing a trouble with the Alt+Tab keyboard shortcut which we use for switching between windows so rapidly. It appears to be a bug as I found in the web but there is a workaround. This is how I overcame the issue after finding the solution from a forum.
We need Compiz-Config-Settings-Manager for this task. Install it by issuing the following commands in the terminal.
sudo apt-get update
sudo apt-get install compizconfig-settings-manager
After the installation, run the command ccsm on the terminal to start this advanced application. There, under the window management tab, you can find a check box called Static Application Switcher which you should check. Then, Alt+Tab combination for switching between windows should work immediately.
 A bug report
 A forum question