@amelia I find that the current version of Viddly, available at https://viddly.net/eng12/how-to-download-youtube-videos , works just fine on Windows 10/64. The user interface is clunkier--as we expect with any "refreshed look" from over-caffeinated web designers. The install file and program folder both pass a Norton Anti-Virus scan.
When doing the install, decline all the "options" it offers. To ensure that there's no new software added to your computer without your knowledge, go to the "Apps" panel in Windows, sort by date, and see if there's anything you don't recognise having been added today.
If you have a Mac, the steps are probably similar.
PS: At one point it asks for your email, but this is for advertising purposes only, not an activation link. I tend to use "[email protected]." He must be getting a lot of spam....