I ran into the same problem recently as well. However, it went away after I installed STOIK Video Converter. You can download it here:
http://www.stoik.com/products/svc/index.html
I have no idea why WMV files no longer buffer after installing that program, but perhaps it'll work for you guys too.
(I know this kind of sounds like I'm advertising for the program, but the only reason I installed it was to convert WMV files into AVI files due to the annoying "buffering" thing. However, since simply installing it fixed the problem, I've never actually even used it.)