The song is a remake of Pure Snows, I think. I don't know for sure if it is that one but it is some remake that they did. I can't find it though.
The song are from 『Kanon・Airピアノアレンジアルバム 'Re-feel'』. You can see it at http://key.soundslabel.com/discography.html, CD KSLA-0010.
These disk have the piano only version of Pure Snows, who it's the song used on these episode. All the disk are piano versions from some know songs from Orito Shinji.
But, it isn't also the "real" song, because, the song on the anime are extracted from these one, but are a bit mixed parts. For example, at minute 15:00 of the episode, the songs "changed" to a piano version of Little Fragments song, from CD KSLA-0003, with name 『Kanon arrange best album 'recollections'』.