Having at least a small bit of skepticism is a good thing... there was an article on Windows Central, the sort where on a slow news day they pay folks to generate [usually low quality] filler. The author talked about the off-line Defender tool from Microsoft, of which he said: "... there's a beefed-up version called Windows Defender Offline. You can run this while not connected to the internet from a USB drive, and it should find those harder-to-kill viruses."
Wrong.
Security software has gotten rather good at spotting files that match their malware pattern databases, so cyber criminals are sometimes upping their game, writing malware that is assembled & functional only in memory, so there are no files to match. They're also increasingly using stuff that's built into Windows, because security software generally gives Windows files a free pass.
That means that your security software needs to be good at monitoring what the processes running in Windows are actually doing, because that's how you catch stuff like fileless malware. That's not saying anything good or bad about Windows 10's Defender or Microsoft Security Essentials -- you can take a look at how they rate in testing at av-test[.]org [they're closer to the bottom than the top]. It is saying that file-based malware detection alone [e.g. Defender Offline], while worthwhile as part of your defense, is not up to the task of protecting you.
"Locky, Cerber Ransomware Skilled at Hiding"
threatpost[.]com/locky-cerber-ransomware-skilled-at-hiding/124441/