1. Now you'd better get a mobile hard disk and delete all the movies on disk C. Don't load the C disk with messy things, just load the system files, otherwise the load on the computer will be too heavy, not to mention the C disk. Even if there is too little space left on the D E F disk, it will affect the running speed of the computer.
You'd better see if there is something wrong with the graphics card. I used to have mosaic problems when I was watching movies, and I didn't take them seriously. As a result, the graphics card burned in a few days. When I took it apart, I saw a layer of dust on the fan, which did not dissipate heat at all. I suggest you clean up the dust on the hardware.
3. Maybe your player's decoding ability is not enough. I don't know what system you are. If it is XP, I recommend you to use KMplayer. If it is WIN7, you can use Potplayer. The decoding ability is very strong, and it is no problem to play high-definition movies.
If you are worried about poisoning, scan it with antivirus software.