Windows 10 includes Microsoft Defender Antivirus in the Windows Security app. You can check whether it is active and scan your PC before deciding whether another security tool is necessary.
Microsoft Defender provides real-time malware scanning and works with Windows Firewall and other Windows Security features. A third-party antivirus may take over as the active provider; installing several real-time scanners together can create conflicts. Antivirus does not replace security updates or a backup.
A scan can remove malware but cannot reconstruct a deleted project or an encrypted document. Keep a separate backup of important files and disconnect the backup drive when it is not needed. If you suspect infection, scan the backup before restoring it to a clean PC.
If Windows Security will not open, definitions will not update, or malware returns after an offline scan, repair Windows and use a trusted recovery route. Do not download an unknown 'antivirus activator' or disable protection to make a suspicious installer work.
Is Defender free? It is included with Windows 10; check that it is the active provider and updated.
Does it back up files? No. Antivirus and backup solve different problems.
Windows 10 already has built-in antivirus protection. Confirm Defender is active, keep it updated, and protect irreplaceable files with a separate backup.