What is an antivirus, how it works, and which ones to use in 2026

What an antivirus does

An antivirus is software that detects and removes malicious programs (malware) from your computer. Malware is a broad category that includes viruses, ransomware, spyware, trojans, worms, and adware — all programs that operate on your machine without your consent and with harmful intent.

The core function of an antivirus is to identify and neutralise these threats before they can cause damage: encrypting your files, stealing your data, making your computer part of a botnet, or silently sending your keystrokes to a remote server.

How it works

Modern antivirus software uses several detection methods simultaneously:

• Signature-based detection: The antivirus maintains a database of known malware "signatures" — unique code patterns associated with specific threats. When a file matches a known signature, it's flagged or quarantined. This is fast and accurate for known threats, but useless against new ones.
• Heuristic analysis: Instead of looking for exact matches, heuristic analysis looks for suspicious code patterns and behaviours — things a file does that look like malware behaviour even if the exact code has never been seen before. This catches variants of known threats.
• Behavioural analysis: The antivirus monitors what running programs actually do — not just what their code looks like. If a program suddenly starts encrypting hundreds of files, accessing the registry in unusual ways, or sending data to external servers, it gets flagged regardless of whether it matched any known signature.
• Cloud-based scanning: Suspicious files are compared against threat intelligence from millions of endpoints in real time. This allows detection of emerging threats far faster than signature database updates alone.
• Sandboxing: Suspicious files are executed in an isolated virtual environment to observe their behaviour without risk to the actual system. Common in enterprise security products.

Why it still matters in 2026

Some people argue that antivirus software is obsolete — that modern operating systems and user awareness have made it unnecessary. This is not accurate.

Ransomware attacks targeting small businesses increased year over year through the early 2020s and show no sign of slowing. The attack vector in the vast majority of cases is a user clicking a malicious email attachment or downloading a compromised file — exactly what an antivirus is designed to intercept.

• Ransomware can encrypt every file on your computer and connected drives within minutes — recovery without backups is often impossible
• Spyware can silently collect credentials, banking information, and business data for months before being detected
• Trojans can give attackers persistent remote access to your machine without any visible symptoms
• Supply chain attacks increasingly target small businesses through compromised software installers

Windows Defender: what's already built in

Windows 10 and Windows 11 include Microsoft Defender Antivirus — a full-featured antivirus built directly into the operating system, enabled by default, and updated automatically via Windows Update. You don't need to install anything or pay anything to have a functional antivirus on a modern Windows computer.

Microsoft Defender has improved significantly over the past decade. In independent testing by AV-TEST and AV-Comparatives, it consistently scores well on protection rates and has fewer false positives than many third-party products. For most small business users, it is a solid baseline protection.

• Real-time protection enabled by default
• Cloud-based threat intelligence through Microsoft's global sensor network
• Ransomware protection via "Controlled Folder Access" (must be manually enabled)
• Behavioural analysis and exploit protection built in
• Integrated with Windows Security Center for a unified view

The main limitation of Defender is that it offers limited centralised management for businesses with multiple devices. If you need to see the security status of 20 laptops from a single dashboard, you'll need a more comprehensive solution.

When to go further

Windows Defender is sufficient for most individual users and very small teams. You should consider going further when:

• You have multiple devices to manage and need centralised visibility and policy enforcement
• You handle sensitive client data or operate in a regulated industry
• You want advanced features: EDR (Endpoint Detection and Response), which provides investigation tools and detailed activity logs for threat hunting
• You have employees who are particularly high-risk targets (executives, finance, HR)
• You need Mac or iOS/Android coverage alongside Windows

State-of-the-art options

A brief overview of well-regarded antivirus and endpoint security products for small to medium businesses. This is not an exhaustive list or a ranking:

• Malwarebytes for Teams: Known for excellent detection of malware that slips past other products. Lightweight, easy to deploy, and good for small teams. Often used alongside Windows Defender rather than as a replacement.
• Bitdefender GravityZone: Consistently top-ranked in independent testing. Strong protection, low system impact, and a good management console for multi-device environments. Well-regarded for SMBs.
• ESET Endpoint Security: Long-standing reputation for lightweight protection with minimal performance impact. Used widely in business environments. Good balance of protection and usability.
• Microsoft Defender for Business: The commercial, managed version of Windows Defender — adds centralised management, EDR capabilities, and cross-device visibility for small business teams at a reasonable price point.
• CrowdStrike Falcon Go: Enterprise-grade EDR technology made available to smaller organisations. More expensive than traditional antivirus, but offers significantly more threat intelligence and response capability.

The right choice depends on your team size, operating systems, budget, and risk profile. For most small businesses starting out, enabling Defender's full feature set — including Controlled Folder Access — and keeping it updated is a meaningful first step at zero additional cost.