Some applications add addition button to make window stay always on top not allowing other open windows to cover it.
This useful feature can be added to almost any application windows via third party tools. One of them is freeware FileBox eXtender. You can download it from here or directly from developer’s site.

This utility adds a little pin icon to all windows. Clicking it makes window to stay always on top. There are some other useful features as well such as roll roll-up window and favorite folders.

Windows XP
Windows Vista
Windows 7