How do you know that the file you are downloading is free from harm to your pc?

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Before, i used to download different files from the net esp songs, and video. I download some applications and programs too for the sake my system stability. But one time, I experienced hassle in my pc. A particular virus that corrupted the system of my pc. So the best way to do is reformat my hard disk. From that moment, I refrain from downloading any files from the net. I just copy some files to my friend to ensure that my pc will be safe from harmful attacks again. Do you experience that too? What can you say about it? Thanks