Some select few people are having an issue with the demo producing a false positive in Windows Defender. Make sure you update your game if you downloaded it yesterday (update then verify file integrity for Steam, delete all files then redownload on itch.io). If you're still having the issue, then you'll have to create an exception for the demo in Windows Defender. Unfortunately custom icons in some Ren'py games set off these false positives - while I believe I've fixed it for most systems, you may still run into this!