Luka, is that Wondershare program that good, cos I've never heard it mentioned. There are basically only 2 Programs Video Aficionados use: VLC and MPC-HC. Maybe a few others. Actually the word Wondershare rears an ugly memory in my head of some nasty virus I got. I'll check the download and make sure it's a safe program. Seems I have heard of it, I can't remember in what light, the light of an infection or the light of a usable program.
The "Video" player app in Windows 8 can play AVI and MP4 files, he can try opening the videos with those. But The Player would not have any bearing on why he can't open his movies. It's sounds like his Permissions.
So, what you would want to do is right-click on the folder the movies are in, then go to Security. You will want to look at the current "Owner" of the folder (and it's subfolders). Set it to "Users"
Then go into "Auditing" and add the Users "user" into that, giving it full permission. Finally, go into "Permissions" and look at the permissions, take a screenshot and post it here. You might want to add Users into there as well, and make sure you are giving it full access.
But the best thing to do is show me the location of these mooovies, and show me a screenshot of the Advanced Security Panel, and I'll see if I can walk you through your Security settings.
Wondershare seems to be clean, it is taking too long to build the subtitle cache. Ok, it's alright, seems to be a decent enough player. I don't think it will solve his problem though, he can try to open his files with it, and see if he has any luck. But check the file permissions first.