Ensuring the antivirus definitions are up to date is critical because new malware variants are constantly being created, and up-to-date definitions are required to identify and remove them. Running a scan without updated definitions might not catch the latest threats. Restoring to a known good backup does not prevent future infections and might reintroduce the same malware if the backup is compromised. Disabling the antivirus might be performed during troubleshooting but is not a preventive measure. Completely reinstalling the operating system would eliminate current malware but does not prevent future infections and is an extreme measure that should only be taken if all other options fail.