Yes. Read their Terms of Service to see the details. Basically, you can host anything you want as long as it is legal.
DH offers "RealAudio and Video (http stream)" as well as QuickTime streaming". Windows Media is not mentioned but you can write them directly to ask about that.
It varies but generally you can begin to work with your account within a few hours after payment. The DNS registrations for your domain may take a few days to propagate over the Internet though, but that has nothing to do with DH.
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