Learn · Windows PC
Check Defender's virus definitions are current
3 min · essential
· HomeOS can check this for you
Why this matters. Antivirus recognises malware using 'definitions' (also called security intelligence), a list of fingerprints of known threats that Microsoft updates several times a day. A PC whose definitions are weeks old can't recognise anything new. Updates normally arrive on their own through Windows Update; this checks they are.
1
Open Protection updates
In Windows Security, click Virus & threat protection, then under Virus & threat protection updates click Protection updates.
Windows Security › Virus & threat protection
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Virus & threat protection updates
Security intelligence is up to date.
Protection updates
Click Protection updates
2
Check for updates
Click Check for updates. The 'Last update' date should be today or yesterday.
Windows Security › Protection updates
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Security intelligence
Last update: today
Check for updates
Last update should be recent
If it won't update, run Windows Update (next guide). Definitions come through it.
Check your understanding
What are antivirus 'definitions'?
A constantly updated list of fingerprints of known malware. Old definitions can't recognise new threats.
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Checked against:
Microsoft: Stay protected with Windows Security
· last reviewed 2026-09