My best tip is to upgrade to a fast HDD, maybe SSD if you can afford it. The HDD velocity and load makes a huge difference when playing larger HD files, specially 1080p and even 720p. Second I think that a dual core is not enough to play the heavier files, unfortunately. I had the same problem a few years back and upgraded to a quad core and could play any larger HD file after the upgrade without problems. Last but not least, a good GFX card is of course also important. Make sure your video card supports HD.
My experience is that codecs, video players etc has little to do with the quality of the output compared to the importance the hardware has. Specially HDD makes a big difference. I recommend that you have the video file on a HDD that is not used by system or your torrent client while you are playing it. Make sure you are not playing it on an external HDD as they usually are quite slow.
I hope I was of some help, and good luck