Ah, misunderstanding -- Trakt.tv is just for
tracking the TV you watch. It gives a history. There's nothing to actually watch on their site.
But if you're interested in the Media PC route -- the software you'd want is Sickbeard, SABnzbd, and XBMC. Sickbeard is like a DVR - you add all the shows you want to watch to Sickbeard. Sickbeard finds the shows available to download on Usenet (newsgroups) and then forwards the download information to SABnzbd. SABnzbd downloads all the pieces of the TV show, puts it all back together, and then plops it into your Media folder. XBMC watches your Media folder and when new shows get added it reads the filename, figures out what TV show, season, and episode it's for, and then queries TheTVDB.com to find out that episode's cast, plot summary, air date, etc. XBMC is the front end that you use to watch all your media on -- it's a GREAT program even if you just have a small collection of shows, it really makes it all easy.
Here's a few articles on the process --
http://forum.xbmc.org/showthread.php?tid=120406
http://www.howtogeek.com/72267/how-to-turn-your-computer-into-a-supercharged-tivo-with-sick-beard/