Why didn’t Windows Defender recognize spyware on my computer?

A reader tells us that he uses Windows Defender to scan his computer for adware and spyware every other day. So far it has reported finding no problems.


However, he writes, he also scans his computer with two other antispyware programs — Ad Aware and Spybot Search and Destroy. Both have reported that he has adware on his system. What gives?


The answer might lie in how Windows Defender and other antispyware products actually define spyware. For example, Windows Defender doesn’t detect cookies. Why? Because many cookies are used for legitimate purposes and Microsoft believes that the appropriate place to manage cookies is through your Web browser.


This is a complicated issue, because new forms of spyware are developed every day, and some components of legitimate programs can meet criteria for spyware, if a program defines spyware broadly.


If you’re interested in how we define spyware, take a look at How Windows Defender identifies spyware and Windows Defender antispyware cycle .


It might also be the case that the version of Windows Defender he is using hasn’t been updated lately. Windows Defender detects spyware by using what we call “definitions.”


These definitions need to be updated in order for Windows Defender to work correctly. Updating them is a snap if you have Automatic Updates in Microsoft Windows turned on, because Windows Defender works with Automatic Updates to automatically install the latest definitions.


To make sure that you have Automatic Updates turned on, click Start, click Control Panel, and then click Performance and Maintenance. Click System, and then click the Automatic Updates tab and review the settings.


Instead of waiting for regularly scheduled updates from Windows Update, you can also tell Windows Defender to check for new definitions automatically before scheduled scans. Here’s how:

1.       Open Windows Defender by clicking Start, clicking Programs, and then clicking Windows Defender.

2.       Click Tools, and then click Options.

3.       Under Automatic scanning, make sure the Automatically scan my computer (recommended) checkbox is selected.

4.       Select the Check for updated definitions before scanning check box, and then click Save.


If you try these steps, but it still looks like Windows Defender has made a mistake, you can report a possible spyware problem to Microsoft.


Windows Defender is no longer in beta! The most recent version is available now at the Microsoft Download Center.

About the Author
Eve Blakemore

Group Manager, Trustworthy Computing

Eve Blakemore is a Group Manager for Trustworthy Computing who delivers consumer guidance around the latest trends in security and privacy. Eve joined Microsoft in 1998 and has worked in corporate and field roles with Microsoft Learning, US Public Sector, Read more »