To play movies I use Totem, (Movie Player in the Gnome menu) but for some reason I could not play the .avi files and .asf files anymore.
Totem is no longer able to play those files after my last (todays) update.
So I decided to try the gstreamer plugins from Debian-Multimedia, unstable (Sid) repository.
I installed the packages gstreamer0.10-ffmpeg-full and gstreamer0.10-plugins-really-bad.
Installing the packages auto-removed the packages i originally used from the Debian repository being gstreamer0.10-ffmpeg and gstreamer0.10-plugins-bad.
Movies play fine again now.