Hey! Help me please!!!
I downloaded and installed Flvto Youtube Converter, but my antivirus wanted to stop this because the app had viruses!
But the support said that their app has no viruses and i can install it.
Help me please, i like this and want to download playlists of music.
But i'm scare about my notebook!

Not knowing what AV product you're using, I'd trust their software more than the "support" department of some piece of software I don't know.

While virus scanners are known to sometimes produce false positives, they do so less often than criminals setting up companies in order to distribute malware that looks legitimate.

If it was this - hxxp://www.flvto.biz/youtube-downloader/ - then the alert was likely for PUA (Potentially Unwanted Application) as the installer is bundled with OpenCandy.
As long as you were careful during the installation and opted out of any other software you were offered, then you have nothing to worry about.