(mabe my reason to update to win7)
Heres a quick howto from another thread:
You should create a playercorefactory.xml in your userdata folder. This works perfectly for me. Except for my mkv's, everything else uses the built-in DVDPlayer. Since I only use DSPlayer for mkv's, this is what mine looks like (I also have MPC-HC in there for testing purposes, but it isn't needed).
<player name="MPC-HC" type="ExternalPlayer" audio="false" video="true">
<filename>C:\Program Files\MPC HomeCinema\mpc-hc.exe</filename>
<args>/fullscreen /play /close</args>
<rule filetypes="mkv" player="DSPlayer"/>
to access other configured players, hit "c" on your keyboard while highlighting a Video and select "Play Using"