If you are getting choppy playback of HD videos, then it could be one of four possibilities:
1. Your PC is not powerful enough to decode and playback that content. HD videos actually needs a decent dual CPU to playback smoothly; say roughly 1.8GHz. That along side a decent graphics card that can assist with the decoding process. An intel HD 2000 should be more than enough for that.
2. You need to update your drivers to maybe solve the solution if your PC too weak to decode HD videos.
3. The video itself was encoded poorly. If this is the case for all HD videos, then either they are all poorly encoded or you have an issue with #1 or #2.
4. The video itself could be fine, but perhaps when download the video became slightly corrupted. This happened to me when I was downloading a video review for the Nexus 7. The video playback was fine when it was streaming, but i wanted to download the video so that I can reference it later. The video playback was choppy. I downloaded it again and it played back fine.