Do you need an antivirus after a reformat?
What is already in Windows is enough for most people. Here are the cases where you do need something more, and the cases where adding one makes things worse.
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Is Windows Defender enough?
Windows 10 and 11 come with Microsoft Defender (Windows Security) built in: real-time scanning, ransomware protection for your folders, SmartScreen filtering of downloads and websites, and a firewall, as one package. It is already on right after a reformat with nothing installed, and it updates through Windows Update, so there is nothing to renew and nothing to pay.
In independent tests from AV-TEST and AV-Comparatives its detection has ranked alongside the paid products for several years now. The idea that "the built-in one is weak" dates from the Windows 7 era. It needs no licence on a home PC or a work PC. That is why this site’s "Basic PC" and "Work PC" templates contain no antivirus — one is already there.
When you do need something extra
- Your employer requires a specific product
- If the company has standardised on one product, use that — with the installer and licence the company provides, not a personal free edition.
- You want a second opinion scanner
- On a family PC where files of unknown origin turn up often, keeping something like the free Malwarebytes — scan-only, no real-time monitoring — next to Defender is fine. Real-time protection from Defender, an occasional deep scan from the other tool: that pairing does not fight itself.
- Security software demanded by a bank or government site
- Some online banking and government portals insist on installing keyboard-protection or firewall components before you can log in. Those are not antivirus; they are conditions of that site. Install them when the site asks, not in advance.
- A Windows version past end of support
- On Windows 7 or 8.1 you are beyond what any antivirus can cover, because holes in the operating system itself are no longer being patched. The answer there is not an antivirus but moving to Windows 10 or 11.
When installing one makes things worse
- Two real-time antivirus products at once
- Each flags the other as suspicious, every file gets scanned twice, and the PC slows down. Install another antivirus and Defender automatically switches off its real-time protection and steps back, so you only ever run one. There is no reason to force both on.
- A personal-use-only product on a work PC
- Several popular free antivirus editions are free for individuals and licensed for companies. Install what you use at home on a work machine and it turns up in a software audit. The "Which programs are safe to use on a work PC" guide lists them. For a work PC, Defender is all you need.
- "PC optimisers", "cleaners" and "virus found" adverts
- A browser page announcing "37 viruses found" and pushing you to install something is always an advert. Real Defender warnings appear in the Windows notification centre, never on a web page. These programs are not protection; they are the thing to remove.
- Antivirus that rides along with another installer
- Free program installers sometimes present "also install XYZ Antivirus" pre-ticked. Untick it and move on. The installer file this site builds uses only vendors’ official downloads, which carry no such bundles.
What to do right after the reformat
Run Windows Update to completion
Defender’s definitions and engine update here. The install image you formatted with is months old, and until it updates it does not know the latest threats.
Open the Windows Security app once
Settings → Privacy & security → Windows Security. Check that Virus & threat protection, Firewall, and App & browser control all show a green tick. A yellow mark means click it and turn it on.
Turn on ransomware protection
Virus & threat protection → Ransomware protection → Controlled folder access. It lets only approved programs change your Documents and Pictures. Off by default; on, it stops ransomware encrypting your files. For the first few days you may get notices that a legitimate program was blocked — add it to the allowed list when that happens.
Settle on a backup
The only way back from something the antivirus did not stop is a backup. Whether it is OneDrive or Google Drive syncing automatically or an external disk, make sure there is a second copy of your documents and photos outside the PC. That is more certain than any antivirus.
About bundled trials and free brands
Many branded PCs ship with a McAfee or Norton trial pre-installed, which expires within weeks and then nags for payment. After a clean install it is gone, and there is no reason to put it back — Defender covers the same ground. Free editions from other brands (Avast, AVG, Avira and the like) work, but come with adverts and upsells, and the free ones have had their own history of monetising user data. If you prefer one of them out of habit, install one only, and Defender steps aside by itself.
Frequently asked
When does Defender scan?
Real-time protection is always on, and a scheduled quick scan runs when the PC is idle. Nothing to do. If you think you opened something dodgy, run Windows Security → Scan options → Full scan once.
A "your computer is infected" window appeared in the browser
An advert. Close the window, do not call the number, do not download anything. If it will not close, end the browser from Task Manager (Ctrl+Shift+Esc). Whether you are actually infected is checked in the Windows Security app.
The antivirus keeps flagging a crack or key generator. Can I add an exception?
Not advisable. Files like that very often carry real malware, and the antivirus has reasons for flagging them. The moment you add an exception, that folder is scanned for nothing.
If I use a VPN, do I still need antivirus?
Different jobs. A VPN hides your traffic; an antivirus looks at files that arrive on the PC. Download a malicious file over a VPN and you are infected just the same.
What about a Mac or a phone?
This guide is about Windows. Android has Google Play Protect, and iPhone and Mac rely largely on how the operating system is built. On any of them, installing apps only from the official store matters more than any antivirus.
Setting up a work PC?
There is a template with only programs that are licence-safe for business use. No antivirus in it, because Defender is the one to use.
Open the Work PC set