Final answer:
The correct next step is to run a full system scan with updated antivirus software.
Step-by-step explanation:
The correct next step for the technician suspecting a computer system is infected with malware is to run a full system scan with updated antivirus software. This activity will help identify and remove any potential malware or viruses from the system. Sandboxing, which refers to running an application in an isolated environment, is typically used for testing and isolating potentially malicious programs, but it is not the recommended next step. Formatting the hard drive would result in the complete loss of data, and ignoring the issue and continuing normal operations can lead to further damage or compromise of the system.