If you have removed any other anti virus suite, go into 'settings', 'privacy & security', 'windows security', this will tell you if need to activate Defender, or if it is already running.
I have Defender with Windows 11. Is this running in the background?
It should be, as the other poster says check it is running after you have removed another product.
The built in Defender in Win10 and Win11 has recently had some really good reports in various AV comparison sites. You shouldn’t need anything else for most people.
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