When the mouse is moved over an open window it automatically sets the focus to that particular window. It does not bring the window to the foreground of the open windows.
Go to: HKEY_CURRENT_USER\Control Panel\Mouse
Modify/Create DWORD Value of Data type REG_DWORD Named [ActiveWindowTracking]
Setting for Value Data: [0 = ActiveWindowTracking Disabled / 1 = ActiveWindowTracking Enabled]
Exit Registry / Reboot