Thank you for all your comments!!!!
I am back to SL now. What have i done?
*I deleted all my Antivir- and Cleaner-Programms. And now I only use the Defender, which is completely sufficient.
*After a while it was possible to login also with Firestorm. But nothing could load. Not the avatar, no gestures, no items... and I couldn't jump to other places. A few hours later, or the next day, things slowly came back to normal. But it actually took some time until the changes to my PC had the desired result. Because the program actually had to recognize the changes first.
# My resume: Microsoft has very good Windows security. We shouldn't and don't have to replace them with other programs. In order to be able to completely delete useless programs from the PC, I recommend "Revo-Uninstaller" ( https://www.revouninstaller.com/revo-uninstaller-free-download/ ).