After getting a phishing scam email, Karen ran a malware scan with Windows Defender and it found a "severe threat" called a Trojan-Downloader. Windows Defender blocked it, but is she still compromised? Leo says that everyone gets those, and it's not a side effect of a virus on your system. So if Defender found one and blocked it, you're safe from it. But Leo also suggests running the thorough scan a second time, and then an online scan from TrendMicro or eset.com. Computers can't have more than one AntiVirus app running on them, but an online scan is a good way to get a second opinion.
But if you're really worried. You can always backup your data, wipe your computer, and then reinstall Windows. It's the only way to be sure, but it's really drastic unless you're certain you've been hacked.
You can also turn on two-factor authentication. But also turn off WAN administration on your router.