Sometimes "stupid.zip" is actually "stupid.zip.exe". Make sure your file explorer is set to so you can see the true nature of the file. A ZIP file that is actually an Executable (.exe) or a Script (.vbs) is almost certainly a virus.
If you are on Windows Pro, use the built-in Sandbox feature which wipes itself clean the moment you close it. 3. Online Scanning
If you are a developer or researcher and must see what is inside:
Run the file in an isolated environment like VirtualBox or VMware so it can't touch your actual operating system.