Alt+tab is the most quickest and easiest way to switch across opened windows and programs but sometimes the size of the small thumbnail in alt+tab switching makes it difficult identify the what is actually going on a particular opened program window.
Alt+Tab Tuner is a free program which helps you increase the size of the thumbnail, and tweak lot more other settings of the thumbnail images of the programs showing while alt+tab switch.
  
As shown in the screenshot of this application, you can change the margins around thumbnails, increase columns in thumbs grid shown, increase the spacing in between thumbnails shown, increase the icon size and shit its location, increase the thumbnail and increase the fading and transparency of alt+tab switch. You can also revert to the old style alt+tab as there in windows xp.
You can get get the layout of alt+tab switching like the one shown below just with minor tweaking.
  
How To Use ?
Using and testing this program is quite simple just, change the options, click apply and then see the changes in alt+tab switching.
Download Alt+Tab Tuner