In basic computer security, what is the most effective way for a typical user to prevent a computer virus from entering and infecting a personal computer system?

Introduction / Context:
Computer viruses and other forms of malicious software can damage files, steal data, slow down systems, and spread to other machines on a network. This question tests your understanding of basic computer security best practices and how an ordinary user can most effectively reduce the chance of a virus entering and infecting a personal computer system.

Given Data / Assumptions:

- We are dealing with a typical home or office personal computer.- The question focuses on prevention of virus infection, not on removal after infection.- Several options describe different maintenance and usage habits, and we must choose the one that directly addresses malware prevention.

Concept / Approach:
Viruses usually enter through untrusted software, infected email attachments, malicious websites, and removable media. The most effective preventive approach combines technical protection with safe user behavior. A reputable antivirus solution with real time scanning and regularly updated virus signatures can detect many threats. Keeping the operating system and applications updated closes known security holes. Finally, avoiding suspicious attachments, links, and downloads reduces the chance of encountering new malware in the first place. General maintenance tasks such as defragmenting disks or cleaning fans are useful for performance and hardware health, but they do not stop viruses from entering the computer.

Step-by-Step Solution:
Step 1: Identify which option directly mentions antivirus software and security updates. Option A clearly talks about installing reputable antivirus software and keeping it updated.Step 2: Note that option A also mentions keeping the operating system updated and avoiding suspicious attachments and downloads, which are common infection vectors.Step 3: Evaluate option B. Turning off the monitor saves power but has no effect on whether malware can enter the system.Step 4: Evaluate option C. Using pirated software often increases the risk of infection, because such software may be modified with hidden malware.Step 5: Evaluate option D. Defragmenting the hard disk may improve performance on some systems but does not remove or block viruses.Step 6: Evaluate option E. Cleaning the fan and cabinet improves cooling but is unrelated to virus prevention.Step 7: Conclude that only option A provides a comprehensive and effective method to prevent viruses from entering the computer.

Verification / Alternative check:
Security guidelines from operating system vendors and security companies consistently recommend three pillars of protection: updated security software, patched systems, and cautious user behavior. These sources emphasize not opening unexpected attachments, not downloading unknown software, and keeping antivirus and system updates current. None of them claim that defragmentation, turning off monitors, or cleaning fans alone will prevent viruses, which confirms that option A is correct.

Why Other Options Are Wrong:
Option B affects only the display device, not the risk of malware. Option C is risky practice because pirated software may contain hidden malicious code. Option D helps performance but does not inspect or block files for malware. Option E relates to physical maintenance and cooling, which has no effect on whether a malicious program can enter the system or execute.

Common Pitfalls:
Many users wrongly assume that only installing an antivirus program once is enough, without keeping it updated or changing unsafe browsing habits. Others focus on general maintenance tasks like cleaning or defragmenting and neglect security updates. Remember that security is a continuous process that combines software tools and smart user behavior. Prevention is much easier than recovering from a large scale infection or data loss.

Final Answer:
The most effective preventive measure is to install reputable antivirus software, keep all software updated, and avoid suspicious downloads and email attachments as described in option A.