What does enabling Periodic scanning by MS Defender do when a 3rd party Anti Virus is installed

Delta Theta 0 Reputation points
2026-09-14T15:02:16.3066667+00:00

I have a third party non-Microsoft Anti Virus, i.e. security suite like Norton or Mcafee, installed. Typically when a third party security provider is installed Microsoft's Defender gets disabled. However in Windows Security > Virus & threat protection > AntiVirus Microsoft Defender options there is the option "Periodic scanning". I have the following questions regarding this.

If we enable "Periodic scanning" what does it do? Does MS Defender do a full system scan or a quick scan of the computer? Is the scan done every day or weekly or some other interval? Is there a way to check out the outcome of the scan? Either via Event Viewer or some other way?

When does the Microsoft Defender download the latest definitions for the Periodic scanning? Is it done daily or weekly or some other interval?

Is this periodic scanning different from the Microsoft Malicious Software removal tool (MSRT) scanning that typically occurs every patch Tuesday after every update?

Windows for home | Other | Security and privacy
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  1. Ramesh 182K Reputation points Volunteer Moderator
    2026-09-25T10:29:44.4733333+00:00

    If the "Periodic Scanning" option (a.k.a. Limited Periodic Scanning) is enabled, it runs a Quick Scan when the system is idle. The scan is triggered by the "Windows Defender Scheduled Scan" scheduled task.

    Here are the relevant logs:

    C:\Windows\Temp\MpCmdRun.log

C:\ProgramData\Microsoft\Windows Defender\Support\MPLog-<timestamp>.log

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  2. CrazyKats 19,840 Reputation points Volunteer Moderator
    2026-09-23T22:06:13.5166667+00:00

    Hi Delta,

    I run Windows Security and Malwarebytes

    side by side with no issue.

    If you open Windows Security, click Virus &

    Threat Protection and you should see this:

    User's image

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  3. Anthony Porter 390 Reputation points
    2026-09-16T23:46:19.56+00:00

    Enabling Microsoft Defender Antivirus periodic scanning allows Defender to run as a second-opinion scanner alongside your primary antivirus product. It does not replace or interfere with the third-party AV, which remains the active antivirus solution.

    The scan schedule isn't directly configurable, and Microsoft doesn't document an exact frequency. Defender automatically performs periodic scans and uses updated intelligence before scanning.

    Periodic scanning is also separate from the Microsoft Malicious Software Removal Tool (MSRT). MSRT is a limited tool that checks for specific prevalent malware families, whereas Defender periodic scanning uses the full Defender scanning engine and signatures for broader malware detection.

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