A computer can easily be converted to a media center for streaming video and music from the internet or locally stored files. Kodi is a great media player that I usually use, but a few Linux distros do a great job of creating a minimalistic OS based around Kodi. For my computer, I use OpenElec:
http://openelec.tv/
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https://www.youtube.com/watch?v=pZoIStYElSo