Well before you reinstalled windows you might had that ac3 codec installed. You should be able to see the formats from left clinking the movie and selecting properties. From there you should see the what codecs are being used.
I personally use AC3 filter. You can get it here:
AC3 Filter download @ free-codecs.com
[EDIT] Just looked some stuff at MS site and found this :
link hope that gives some inside how WMP need some plug-ins to play DVD.