Since you're using the dreamspark, this is most probably due to your installed 3rd party codecs doing something wrong. Your best bet is to try other H.264 codecs (and disable the ones you are currently using in the Options->Tools->Compatibility menu).
Because these 3rd party codecs are not Microsoft's technologies, I'm afraid we can't provide more help on the matter. Feel free to contact the people who wrote this 3rd party codec for more help.
