Can Windows Defender catch all viruses?

It can't. Windows Defender is decent for basic, free protection, but it doesn't catch everything, you know? The real deal is that no antivirus can catch 100% of viruses, because new threats pop up every day and hackers are always creating new malware that bypasses known signatures.

Defender is good for everyday protection, especially if you don't mess with sketchy downloads or weird sites. But if you download a lot of files, use torrents, go on suspicious forums, then the risk of it missing something goes way up. For example, sophisticated ransomware and advanced trojans can slip through undetected because they use obfuscation techniques or exploit zero-day vulnerabilities that haven't even been discovered yet. I know people who got viruses even with Defender turned on because they opened an email attachment that looked legit but had some really clever malicious code embedded in it.

The right way to protect yourself is to combine multiple layers: yeah, Defender on, but also common sense (don't click random links, don't download pirated programs), keep Windows updated, use a solid password manager, and maybe add a more robust antivirus like Kaspersky or Bitdefender if you deal with lots of files. Even me, when I'm running around from morning shifts to nights working at the barracks, I still think it's important to keep my laptop properly protected. Oh, and running an additional scanner like Malwarebytes every once in a while helps a lot to catch stuff that slipped through.

I agree with what was said above. In my 26 years of experience, I've seen a lot of people relying too much on a single protection program and then getting a nasty surprise. Windows Defender sits around 85-90% effectiveness in independent tests, which is reasonable for someone who doesn't want to spend anything, but the truth is new malware pops up constantly and no software can catch everything. If you use your phone or computer for important stuff, like banking or sensitive photos, it's worth combining Defender with another more robust antivirus and, most importantly, having common sense: don't open strange attachments, don't download from suspicious websites, those basic things really.