Through much trial and error, tested upon countless newbies (who don't know how to read the "sticky" important threads) we have found a simple and quick solution that will fix
at least 98% of all problems that we see. I expect it will work on your problem as well.
Uninstall all your codecs. Trust me.
All of them:
Uninstall DivX.
Uninstall XviD.
Uninstall ffdshow.
Especially uninstall any Codec packs,
They are Evil!
Download and install ffdshow. You'll find it listed as [Level 1] on the
Media Configuration Guide.