Hmm.. the times i've had trojans, they've been files that i just had to change the extension of, then they dissapeared and the files could be deleted. You should download Avast! Antivirus. Just google it, im unsure of their site. I've had it for along time and it gets rid of trojans quick and easy.
I never, ever download codecs. You shouldn't either. Nor 'licenses'. If you get a suspicious video (dont work properly) delete it immediatly.