Which of the following is NOT a type of computer virus?

The selection of the Trojan virus as the answer indicates a misconception about how viruses are classified. A Trojan virus does not fall under the category of viruses like the others listed. Instead, it is classified as a type of malware that disguises itself as a legitimate program but actually performs malicious actions once executed. Unlike traditional viruses, Trojans do not replicate themselves or spread from one infected file to another.

In contrast, document viruses, overwriting viruses, and companion viruses are indeed recognized types of computer viruses. Document viruses infect files associated with specific applications, typically targeting documents created with programs like Microsoft Word. An overwriting virus is designed to overwrite existing files, potentially destroying them in the process, while companion viruses exploit the way operating systems handle file execution to infect a system through a secondary file. Understanding these distinctions is crucial in recognizing the various types of malware and their behaviors in computer security.