LS it definitely sounds like a codec issue.
What codecs have you installed I usually use the K-lite one because it covers several codecs.
Here is a link to the K-lite codecs (I hope it is okay to post it here.)
http://www.free-codecs.com/download_...=7129&s=775&r=
Codecs are a pain in the rump and sometimes it just takes finding the right one.
If that doesn't work let me know