Final answer:
A technician will most likely need to perform a clean operating system installation if a backup has not been performed in the event of a hard drive failure or a severe virus infection.
Step-by-step explanation:
If a technician has not performed a backup, there are two occasions where a clean operating system installation is most likely needed. One is in the event of a hard drive failure, where the existing operating system and data become inaccessible. The other occasion is when there is a virus infection that has severely compromised the operating system, making it necessary to reinstall from scratch.