An HTPC is not just a pc which plays movies. You can do lot more than that. For example you can write/copy DVDs, DIVX, capture Video, listen music, surf the internet, play games and if you combine that with a VCR then you can convert your old video tapes to DVDs and keep them unaltered for many years.
   Except the applications you can run on an HTPC it supports all AVI formats and subtitles. Also it is sure that you are will be able to watch movies with future formats which a stand alone player may not support. The HTPC is programmable. That's why it is so special.
Look and performance costs enough. What can I do?
  A powerful HTPC costs more than 1000$. Although there are some stylish low cost solutions, which we are going to introduce you, with strong enough cpus in each case.