At the moment it's a tie between:

1) The way it periodically goes "You know what, FUCK YOUR PREFERENCES" with the result that explorer goes back to sorting folder contents by name instead by type; switches back the ridiculous slideshow display from the more sensible 'details' and decides to have all your media open in the spectacularly shithouse Windows Media Player even though you've previously specified that media files should open in any program BUT WMP.

and

2) Audiodg.exe - which I see has survived the port to Windows 7, proving once and for all that you can have as many degrees and as much talent in your field as you like, but at the end of the day at least one cretin will still earn more than you working at Microsoft.