Vista Tutorial - video driver or codec problem

I'm running Vista Ultimate on a Gateway ML6721 with an Intel 965 video
chipset. With my current video driver, I can view wmv files at various
resolutions and full screen, both regular wmv decoded with the wmv8 codec and
newer files decoded with the wmv9 codec. After upgrading to the current video
driver via windows update, files decoded with the wmv9 codec play fine.
However, files decoded with the wmv8 codec only play at 100% resolution, they
will not play at 50%, 200% or full screen. It also doesn't matter if I'm
using an external monitor. Reverting back to the previous driver corrects the
problem. Would this be a video driver or a codec problem? I'm guessing the
former as updating the video driver should not affect installed codecs.
Thanks for any comments. I'm mainly trying to better communicate this issue
to Intel and Gateway.