It won't make any difference. For playback all the codec packs do is let you play files with specific codecs, or not.
It's likely the file you have is simply low quality, lots of stuff on the net claim to be HD just because they use 720p or 1080p resolution but the bitrate is so low that they look awful.
If you want to know what's going on with a media file install MediaInfo, then you can right click any vid and see exactly how it's been encoded
http://mediaarea.net/en/MediaInfo