Andy,
If you install the 'full' K-Lite codec pack it will allow you to play all types of media files including .MKV - the common container type file that High Def films use.
It's freeware and works well, you can launch the file with the media player incuded in the pack or use your Windows media player (the codecs will still work)
You can download it from lots of hosting sites,
here's one