I apologize in advance if I accidentally missed a rule about posting in the tutorial section...
I used to have problems playing back HD content at 1080p and 720p on my little msi wind packed with an intel atom processor. I mean... it sounds impossible right?
Well I stumbled across a codec called Coreavc codec (corecodec)... It is honestly the best thing ever.
I can play a HD movie on my 1.6ghz atom processor. That's just amazing people.
And on my newly built htpc... well lets just say it runs so smoothly and almost instantly.
but anyway, if you have trouble playing HD content on your computer, definitely give coreavc corecodec a try. It might just do wonders.