Windows 7 All video files have turned into .pptx files

When you right-click on one of those files and select properties, what does it say about it?

Change opens with to use a media player.

This is what I get, since it's in Norwegian I will translate.
This is a video I downloaded a while back from youtube, it used to be a video file but suddenly yesterday after I did a little cleanse of malware and such and did a restart of my computer ALL of the video files on my computer have turned in to .PPTX files, before I changed it (I was dumb enough to not take a screenshot before I changed) the 'open with' said DDL file, but even if I change it to open with VLC it doesn't work, VLC does open but it just stops like it usually does if I play a video file that is 0mb but you can clearly see in this picture that the file itself is 368mb.
Not really sure what has happened but I have never come across having done my weekly malware check that my video files get changed like this and I can't seem to find any information on something like this happening to someone else because all I get is a bunch of forums and posts about PowerPoint and not being able to open .PPTX files with that.

Just out of curiosity, what happens if you open them with VLC?

Well the .LLL part of a file name is called the shell extension. It's only used by Windows explorer to look up that extension in the registry and display the application as the icon and open that file with said program. It doesn't mean the file has been converted from a video to a pptx that's not really possible in most cases. You should be able to change the extension back to the appropriate one and Windows should identify and open the file with the correct application. I can't speak to why your phone changed the extensions on the files.