This can be quite useful when you have many open windows
on your screen :


- HKEY_CURRENT_USER
\Control Panel\Mouse\


Locate under this key the entry :

ActiveWindowTracking

(Double click it and set its value to 1 to enable it.0 will disable it.)

If there is no such entry you need to create it.
(Right click in right pane , new , Dword Value)
Name it:ActiveWindowTracking

This will set the focus automatically to that window over which you
move the mouse.Note that this will not bring the window to the foreground.
Close registry and reboot.