Another sort of devilish thing that can and does occur with these online converters is a window that takes up your entire screen demanding some sort of thing. You cannot close this window. You cannot stop the computer normally. You have to turn the power off altogether. And then when you boot up again, you may find that your browser goes back to that same window, requiring a second shutdown.
AVG does not stop this sort of thing; the only way is to close the window before it completes.
AT this time, I can't really recommend an online converter. When I feel the risk is worth the gain, I tend to go to https://www.flvto.biz/
However, its final step has been showing these problems lately.