RadiAnt is OK - it's got a trial license model but when the license expires you can request an extension for free (big WinRAR energy). There's nothing as good as Horos that is similarly free and open source, sadly.

What you may prefer to do is use the excellent pydicom library for Python, which loads dicom files as numpy arrays, allowing you to convert to any other file format you fancy, whether that be image or video.