I consider my Antivirus (Windows Defender) to be my Second Line of Defense, because
it's more important to keep your OS, third-party software and knowledge updated.
- User clicks (You)
- Updated OS & Browser & Internet-facing Apps
- Antivirus
- ...
If I were to install an on-demand / scanner, these two would be my only options:
- ESET Online Scanner [Link]
- Emsisoft Emergency Kit [Link]
The latter being portable and available for off-line scanning, whereas ESET requires an Internet connection, hence the name 'Online Scanner'.
Remember that Windows 10 users on the latest stable build benefit more from the built-in security features, than that of users on Windows 7/8.1.
In the worse case scenario, nothing beats data backups and system image restoration.