If you’re using Windows Defender as your primary antivirus, then it’s pretty easy to start an offline scan – simply go to the Window Security app > Virus & threat protection > Scan options > Microsoft Defender Antivirus (offline scan)

What this will do is reboot your PC into a minimal environment that’s just enough to allow Defender to start up and scan your PC, and in doing so it will hopefully ensure that there are no resident threats that are affecting the results of the scan. To quote Microsoft:

https://support.microsoft.com/en-au/windows/virus-and-threat-protection-in-the-windows-security-app-1362f4cd-d71a-b52a-0b66-c2820032b65e

Microsoft Defender Antivirus (offline scan): Uses the latest definitions to scan your device for the latest threats. This happens after a restart, without loading Windows, so any persistent malware has a more difficult time hiding or defending itself. Run it when you are concerned that your device has been exposed to malware or a virus, or if you want to scan your device without being connected to the Internet. This will restart your device, so be sure to save files you may have open. Microsoft Defender Offline will load and perform a quick scan of your PC in the Windows Recovery Environment. When the scan completes, your PC automatically restarts.

If however you’re running a third-party antivirus app, then this option may not be visible in the UI.

In this case, you can simply drop into a PowerShell prompt and type in

Start-MpWDOScan

This will reboot your device and begin running the offline scan. Once the offline scan has completed it will reboot once more and you’ll be back in Windows.

Note that you will need your Bitlocker key to unlock your hard drive when booting into the offline scan, so make sure you have it handy. To find your recovery key, open a CMD prompt and type in:

manage-bde -protectors -get C:

This will show you the recovery key, listed as the password.