I run HTPC (Home theater PC) which I use for all my media purposes and choose Linux and MythTV as my front end as it is more stable and smaller that Windows Media Centre which gives me more space for media storage.
The platform I use is the ViaITX platform which has lower power demands and because of this requires minimal cooling which ensures it is near silent and not interrupting the experience of the media playback. When taken into consideration the hardware media decoder available on the ITX platform it makes for a decent small HTPC.

So small in fact you can build HTPC's such as this :



bodywork removed showing PSU and motherboard



DVD-RW exiting from the rear seat.



Ive done computers in many things such as, mini-motos, dustbins, motorcycle helmets, model cars [ including cossies and focus - people love having computer that is a replica of their car for some reason ] and more

A media pc is as complicated as you wish to make it...just plan well and you will have no problems