i built a small win7 box for this task. i picked the components specifically for no fans because it will be on all the time (it's basically a netbook motherboard - laptop PSU, no CPU fan, just heat sink) so it is silent. It's not really the simplest or cheapest of solutions but I like a new toy...
I run it in conjunction with a NAS which the media PC maps drives from (I am risking disk failure as it's a cheap NAS but the plan is to upgrade.)
I mostly use sabnzbd to get all my content, so i can access the interface through my brower without going on the PC as it sits under my TV with no Keys/mouse, there are plugins for my PC/phone so I can set downloads off from remote PC's in 1 click, quite cool.
The next task for sabnzbd is to ensure 'watched' folder works properly - I think I can get it to look in my dropbox for .nzb files so hopefully this will solve the issue I have of no memory - someone tells me a TV show is great but I'm not home - I want to get my phone, stick the .nzb in my dropbox then have my media PC pick it up and start downloading all ready for the time I get home. I think theoretically it should work but I've not had a chance to check the dropbox app on my phone will do this yet. Fingers crossed.
I get music by sticking a shortcut to itunes in the startup folder - then playing music through my speakers controlling it with the apple remote app - no mouse of course!
I play my TV/Films using windows media centre as it's just a win7 box. I have all the plugins sorted so i dont care about converting stuff I just take it as it comes. Might investigate that when space becomes an issue. I also have some WMC plugins to get onto xbmc/youtube/iplayer etc. I bought the official microsoft remote to control it, seemed the easiest way. Amusing that when I tried it out it started sending my xbox mental. Didn't realise it was going to do that... ended up taping over the sensor.
I also have a TV card so I can just watch normal freeview, although I never really do. Obviously WMC will record too but I never bother with that either.
As I said it's not the easiest solution to get going but I like that if I think of something for it to do, I know I can get it setup somehow...