I just noticed that a strange icon is appearing and quickly disappearing in my system tray whenever I click on an OGG Vorbis file. Can anyone tell me what this is and what it's doing when I click a file? It doesn't appear when clicking on MP3s.

Have you installed DirectShow Ogg Vorbis filter? It usually displays that icon when you play Ogg Vorbis files in Media Player. :D

That's the Ogg Splitter tray icon. If you click on it, go to Properties -> Ogg Splitter there's an option to disable it. It just gives you easy access to the properties of the various filters being used to play the file back.