Windows 7 (and Vista) show a preview window thumbnail when you hover your mouse over a minimized or hidden window icon in the taskbar. This is useful feature but it can become resource hungry if you open many windows at the same time. You can disable this feature if you do not like it. This is how :
- Press the key combination Windows logo key + R to open the Run dialog.
- Type gpedit.msc in the Run dialog and press Enter. This would start the Local Group Policy Editor.
- In the Local Group Policy Editor, navigate to User Configuration → Administrative Templates → Start Menu and Taskbar as shown
- Scroll down the list on the right side pane and find Turn off taskbar thumbnails.
- Double-click on Turn off taskbar thumbnails and choose Enabled and click OK to save the settings.