A computer which is connected to the internet needs protection from all sides, as even if the user has nothing to hide, their system can still contains lots of personal data. Pictures, passwords, use history, all can be tracked and breached through many ways. Most cases of security breaches are minor, adding bloatware or malware to a computer, but the serious threats use the same weaknesses in a system to infect. Having pop up ads may not seem like the worst thing, but this could've easily been spyware or ransomware.
As always with these things, the first line of defence will always win the battle; an encrypted off-site backup of the system. If anything was to happen, then the only fail-safe method of patching security is to have a complete copy of your system and files which you can back up from. It's as simple as downloading an image.
The most common way for a malicious user to breach a pc is through the use of phishing scams. These pages appear to be normal pages you would see, such as an email from PayPal or HMRC, but clicking a link causes a download to your computer filled with malicious software. The biggest security flaw for any computer is having poor malware protection, such as through Kaspersky or BitDefender. Most attacks occur this way, and it is famously how the Locky strains of ransomware infected and shut down thousands of NHS computers in early 2017.
Firewall is the next biggest flaw in most peoples security. A firewall is put in place by default in most large OS' (Windows, Mac, Linux/Ubuntu) and this is often good, but is still vulnerable to a targetted attack. Make sure firewall software is constantly updated, as a firewall is there to protect from direct intrusions, and to monitor SQL packet injection. A good firewall such as through Norton or BitDefender will protect a computer from most threats.
After firewall protection, any attacks that occur are usually targeted, and not simply a 'wide net' method of intrusion. It's important to note that if an attack does lock your computer, a backup will always save it. This is where the real details of attack prevention start, with a complete security package as the best option for protection here. BitDefender is great as an antivirus and firewall, but it cannot modify the registry, or control which ports your system allows connections through. IP spoofing has decreased drastically in the last few years, as firewalls began to recognise patterns, but it still happens to machines which appear local.
One of the massively underrated aspects of cyber security is to use an adblock extension while browsing; popups are rife online and these can often lead to malware. Using a program like uBlock Origin is likely the most suitable answer here, as popups do pose a risk. It can always be disabled for sites' the user wishes to support, and enabled when browsing lesser known websites.
Eduction is also an incredibly important step into protecting a system, but this is easier said than done. Bad-browsing habits are common in most people, and also lead to intrusions. Simple things such as not trusting unknown downloads or websites should be easy to teach, but alot of people simply haven't experienced many download sites, so don't know which ones to trust.