What the argument says
Lewis traces the history. Windows had no real built-in anti-malware before Vista, and even then it only covered spyware. A more complete antivirus arrived with Windows 8, but few people used that release. Most Windows users didn't get built-in protection until Windows 10 shipped in 2015. By then, installing an antivirus right after setup was a habit.
His case rests on three points:
- Windows Security covers real-time protection, ransomware protection, scans, firewall and network protection, and device security.
- AV-TEST results since 2020, as Lewis tallies them, show 33 perfect protection scores out of 40 and a 5.91 average. He reports a 99.74% average zero-day detection rate against a 99.34% industry average. He also reports 31 perfect performance scores out of 40.
- Third-party suites tend to be noisy, with pop-ups, discount ads and feature nags. Some also add overhead.
These totals are Lewis's own tallies. I haven't recalculated them from all 40 reports, so treat them as his figures, not independently verified numbers.
What Microsoft says
Microsoft's documentation gives the strongest practical backing. Microsoft says Defender Antivirus includes real-time, behavior-based and heuristic protection, integrated with cloud-delivered protection. The same documentation says that if you install and turn on another antivirus app, Defender turns itself off, and it turns back on if you uninstall that app.
Microsoft's support FAQ is blunter about running two products at once. It recommends against running other antivirus or antispyware alongside Defender. It says multiple real-time products can hurt performance and can cause update or install errors, such as 0x80070643. It also warns that a device is vulnerable if Defender is disabled and nothing else is protecting it.
Some caveats apply to the "built-in is enough" claim:
- Windows Security is an interface over several protections. Smart App Control, Controlled folder access and SmartScreen are separate features, and not every optional protection is on or configured the same way on every PC.
- Managed PCs may have these settings controlled by policy, so local changes may be unavailable.
- Enterprise environments use different tooling, such as Microsoft Defender for Endpoint, and this advice is about home users.
What the latest lab data shows
The most recent consumer round in the research ran in July and August 2026. AV-TEST's report on Defender Antivirus (Consumer) 4.18 on Windows 11 Professional shows a protection score of 6.0 out of 6.0. Its performance score was 5.5 out of 6, not perfect. In that round, 100% of the 297 zero-day samples and 100% of the 12,543 widespread-malware samples were handled.
The earlier May–June 2026 report gives more detail on what that performance score contains. On a standard PC and a high-end PC respectively, AV-TEST measured these slowdowns for Defender:
| Activity | Standard PC | High-end PC |
|---|---|---|
| Launching popular websites | 14% | 7% |
| Launching standard applications | 12% | 12% |
| Installing frequently used applications | 30% | 60% |
| Downloading frequently used applications | 0% | 0% |
| Copying files | 4% | 1% |
The July–August report shows a similar pattern. It lists a 12% slowdown on a standard PC when launching websites and 11% when launching applications. Installing applications slowed 15% on a standard PC and 44% on a high-end PC.
Two points follow from this:
- Installation really is Defender's weak spot, as Lewis says. The June 2026 high-end figure of 60% is large.
- Defender is not impact-free elsewhere. Website and application launches show double-digit percentages in some test cases. These are measurements from a specific lab, not predictions for your PC.
Lewis explains the install gap by saying Defender scans every file a new program writes. He says many rivals use reputation and allowlists instead. I couldn't verify that mechanism independently. It is a plausible explanation, but it is his, not AV-TEST's.
Defender isn't ahead of everyone
AV-TEST's summary for the same round gives several third-party products 6/6/6. A WindowsForum analysis lists Avast Free Antivirus, AVG Antivirus Free, Bitdefender Total Security, Malwarebytes Premium, McAfee Total Protection, Norton 360 and others at full marks. Defender tied them on protection and sat half a point behind on performance. So the claim that third-party antivirus often slows your PC isn't borne out by that round, in which many paid and free suites performed at least as well on speed.
The wording matters here. Defender's 17.5 out of 18 meets AV-TEST's TOP PRODUCT threshold. It is a top-tier result, but not a perfect one. The manufacturer history shows full marks in February and April 2026, then 5.5 for performance in June and August. AV-TEST's earlier history also shows a rougher 2022 period, with a performance score of 4.5 in August 2022. That fits with Lewis's mention of a bug that was later fixed, though I haven't confirmed the bug's details independently.
The fair reading is that Defender's protection is consistently top-level and its speed is good but not best-in-class. It is a strong baseline, not a clear winner.
Where third-party suites still make sense
Lewis notes that the free antivirus market has consolidated under one company. Today's suites compete on extras such as identity protection, phishing protection and dark-web monitoring. Those are separate from antivirus engine quality. One 2026 comparison names cross-platform coverage for Macs, Androids and iPhones as the strongest argument for a paid multi-device suite. That comparison also says Defender offers little household management and no help once a machine is badly infected. Treat it as one outlet's view, but the point about extras is reasonable.
Consider a paid suite if:
- You need protection across non-Windows devices from one dashboard.
- You value identity monitoring or family controls.
- You have a specific reason to prefer a vendor's engine.
Just don't run two real-time engines at once.
Check your own PC
Before you uninstall anything, confirm what is protecting the machine. Microsoft's documented route is:
- Open Windows Security and go to Virus & threat protection, then Manage settings.
- Confirm Real-time protection is on.
- Microsoft recommends turning on Cloud-delivered protection and Automatic sample submission for optimal protection.
- To see which antivirus is active, check Who's protecting me? and then Manage providers in Windows Security.
- To scan a single file or folder, right-click it in File Explorer and choose Scan with Microsoft Defender. On Windows 11 this may be under Show more options.
If you're moving to Defender from a third-party product, uninstall the old product. Microsoft also suggests running the Microsoft Safety Scanner to clean any malware. On-demand scanners like it can sit alongside real-time protection because they only run when you ask them to.
If you see error 0x80070643 during updates or installs and have two security products installed, overlap is one possible cause according to Microsoft. It is not the only cause of that code.
Bottom line
For most home users on a supported, updated Windows 11 PC, Defender is a credible default, and installing a second always-on antivirus isn't automatically safer. Microsoft itself warns against running two real-time products. Lab results back the protection claims.
The "slows down your PC" claim is shakier. Defender's own performance score dipped to 5.5 in the latest rounds, and several third-party products matched or beat it. Installs are its weakest area, though only some people do those often.
Lab scores aren't guarantees, and antivirus alone doesn't make anyone safe. Updates, careful downloads and a backup habit matter just as much.