Windows 7 All video files have turned into .pptx files

[AlphaDawg]

As the title states all of the videos on my computer have for some reason turned into .pptx files, even the videos of my family vacation that I transferred to my computer from my phone have for some reason become .pptx files.
I have tried renaming a video and removing the '.pptx' part and it turns back in to a regular video file but when I try to play the video it won't play, I have also checked the size of the files and they are the same size they always were but for some reason they have all turned into powerpoint files.
Is there a way I can get all my videos back to normal like they were before?

 

[livix07]

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.

 

[AlphaDawg]

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.

 

[livix07]

This behaviour is strange.
I would definitely boot the computer into Safe Mode and scan it with antivirus. Most of the viruses are not active when the computer is running in Safe Mode and this increases the chances that the antivirus will detect them. You can download Malwarebytes and install it while in Safe Mode and scan, see here.
These are the steps I would take:

1) boot into Safe Mode
2) run Disk Cleanup utility to remove unnecessary files (this does not remove your personal files)
3) install Malwarebytes and scan computer
4) open Command Prompt and run chkdsk /r command to check the file system and repair it if broken

 

[bochane]

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

 

[Neemobeer]

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.