What is a primary characteristic of viruses in a PC?

A primary characteristic of viruses in a PC is that they replicate by modifying themselves. This is a defining trait of computer viruses, which enables them to spread and infect other files or systems. When a virus infects a host file, it typically attaches itself to that file and can then create copies of itself. This self-replication often results in the alteration of the host file or the creation of new, infected files, leading to potential damage or disruption to the system.

The ability to modify and reproduce is what distinguishes a virus from other types of malware, such as worms, which can self-replicate and spread independently across networks. In contrast, the options mentioning starting on their own, spreading over networks, or not affecting system performance do not adequately capture this critical aspect of a virus's behavior. While some viruses can indeed start without user assistance or might spread over networks, it is their self-replicating nature through modification that is central to their function and impact.