I think you'll find you need to have Vista for Mpeg4, H264 support. If you
do try any codecs you find on the net, at least use system restore to ensure
you can get rid of them if they cause more trouble than they solve.
Nero & supports H264, as does Apple Quicktime (which I would never
knowingly install on any of my computers). AFAIK, Microsoft has not
released a licensed H264 decoder for WMM, since WMM is free.