Ah yes! it was the Nvidia driver. I had installed the newest available for my card (591.74).
Uninstalled that and Windows installed it's default driver (576.52). All good now
Thanks everyone!
You can do single or batch file conversions - wizard driven and very straight forward. I would start with one or two files to test. Since the extension will be changed for the output (instead of EXAMPLE.F4V it will be EXAMPLE.MP4), you can convert from F4V to MP4 within the same location.