Get one of the inexpensive android media players. Quad core CPU, 1 or 2 GB RAM. Then get your favorite phone media player on it. BubbleUp, Kodi, VLC, whatever.
Even Windows itself contains basic functionality (first with WMP 12 unless I am mistaken)... When your TV is connected to the network, just rclick a file and pick "Play to -> (your TV)". I've got verified it works well with Philips TVs (3 different models, each in different model line, one pretty lowend). It does the work, but for more advanced things, I concur with Jester Maroc.
I tried PLEX but the problem is that the media PC I was using had no internet connection which did not make PLEX happy. Kodi seems to not care so much about being offline.
Get one of the inexpensive android media players. Quad core CPU, 1 or 2 GB RAM. Then get your favorite phone media player on it. BubbleUp, Kodi, VLC, whatever.