Perhaps more helpfully, checking the command line gives me: (line break inserted for your reading pleasure)
Googling AB8902B4-09CA-4BB6-B78D-A8F59079A8D5 shows this ProcessID to be linked to thumbnail caches. The solution on the internet seems to be to give up on thumbnails entirely. Surely one can do better than that?
By using the handle list feature of Procexp I saw the process (select process, choose View → Lower Pane, look for "File" type entries) I saw in particular that the process had an open handle to this file:
...so I imagine that the issue with thumbnailing videos. Now, Tom Wijsman has already discussed how to handle misbehaving thumbnail providers, but here ShellExView shows no non-Microsoft thumbnailing services!
My current estimate is that this must've been a problem with
sigh.wmv in particular, as I generated this file by converting another video file to Windows Media Video through VLC; this file opened just fine in Windows Movie Maker, but the exporting of the relevant project hung at some point in the video.