Downloads from unknown sources - risk?

Hello,

occasionally there are files from unknown sources that I would like to download. More specifically, mostly it is about video-files (.mp4, .mkv etc.) and video-files packed in .rar-files.

 

My question is: What kind of a risk would I take If I was to download these files while having Norton 360 installed and updated on my computer. Definition of risk: malware in general --> trojans, viruses etc. What are the chances of "contaminating" my pc with any kind of malware?

 

Thanks in advance for your efforts and best regards,

Andi