1. As you get the message when accessing videos, it can sometimes be cured by going to Folder Options > View Tab and put a tick in ‘Always show icons, never thumbnails‘.
2. It is sometimes caused by a third-party extension. ShellExView can be useful to see which shell extensions have been introduced into Explorer. Read about it then scroll down near to the bottom to download the x86 (32-bit) or x64 (64-bit) version here
http://www.nirsoft.net/utils/shexview.html When you run it, click or tap on the column labelled Company to alphabetically sequence the entries. You have to assume that the Microsoft entries are bona fide, so concentrate on the third-party entries. Is there anything listed that is either unnecessary or unknown? If so, highlight it and click or tap the red spot (top left) to disable it. There shouldn’t be too many of them but you may find it easier to disable them all, then re-enable them (green spot) one at a time and try File Explorer until the culprit is found.
3. If the issue persists, read about and then download, unzip and run Process Monitor version 3.1 (it doesn’t need installing), from here
http://technet.microsoft.com/en-us/sysinternals/bb896645.aspx Because of the huge number of activities, it can be made easier to understand by de-selecting some of the activities. The activities can be toggled on/off by clicking them on the right-hand side of the Toolbar, they are Registry, File, Networking, Processes/Threads and Profiling. Also, click or tap Autoscroll on the Toolbar to see the current real-time events.