The default photo viewer on Windows 7 does not play animated gifs, even though this feature was present 10 years ago in WinXP and arguably animated gifs are in even wider usage now.
They claim they didn't add support because "the GIF format is not widely used for photos"[1]. But it is very obvious they did this to promote the usage of IE. I wouldn't be surprised if they purposely removed already written code just to cripple the Windows Photo Viewer.
I would like to shame Microsoft engineers who frequent HN into fixing this.
[1] http://social.technet.microsoft.com/Forums/en/w7itprogeneral/thread/b0e84027-9753-46cb-94b8-b099c0a60e1d