But I don't want to spend any money on an antivirus either
Where did you download it?
If free is too expensive, then I can't help you.
Did you download it from the official website? Just do a scan with Windows Defender. In a pinch, you can also use Bitdefender and start a free trial.
https://de.malwarebytes.com/...bytes.com/
also excellent in the free version
Windows says I don't have one, how can I get the free trial?
Then you most likely don't have one either. Windows Defender is really good. Here you can press BitDefender, for absolute security: https://www.bitdefender.de/Downloads/
Which of these variants should I download?
The Colonel
Windows Defender is sufficient, otherwise start a full investigation and only download Optifine only on https://www.optifine.net/home.