When you have many windows open on your Windows desktop, to activate a particular windows, you have to click on it. But if you wish, you can activate windows and steal focus by simply moving your mouse to the windows and hovering over it.
Activate a window by hovering over it with your mouse
To do so, open Control Panel > Ease of Access Center.
Click on Make the mouse easier to use.
Now under Make it easier to manage windows section, check Activate a window by hovering over it with the mouse.
Click Apply/OK and Exit.
Now if you see, an open windows will get activated, when you simply move your mouse pointer over it.
NOTE: In Windows 8 however, this could have a side-effect. You may find that when you go on to open the Charms Bar, the Charms Bar may close immediately after it is displayed. If you face this issue regularly, you may want to reverse and deactivate the ‘Activate a window by hovering over it with your mouse’ setting again.
While at the ‘Make the mouse easier to use’ section, you can also use it to change settings, so as to:
Need more Mouse tips? read this post on Mouse Tricks For Windows.