Should I have an antivirus other than Windows Defender?

IMO Windows 8/10 Defender is just as good as any other free antivirus solution (and probably easier to use for the novice) without bundled toolbars or nagging popups.

• Is Windows Defender Good Enough
• Microsoft: Our commitment to Microsoft antimalware

However, if you have Windows 10 Anniversary update, it includes Limited Periodic Scanning and allows you to use a third party anti-virus program as your primary protection. You may want to read Choosing an Anti-Virus Program.

Although Windows 8/10 Defender provides some anti-malware protection...it is weak, meaning it does not provide comprehensive protection and cannot prevent, detect and remove all threats at any given time. This is true for most anti-virus solutions. Anti-virus and anti-malware programs each perform different tasks as it relates to computer security and threat detection. Essentially, they look for and remove different types of malicious threats.

In simplistic terms, Anti-virus programs generally scan for infectious malware which includes viruses, worms, Trojans, rootkis and bots.

Anti-malware programs generally tend to focus more on adware, spyware, unwanted toolbars, browser hijackers, potentially unwanted programs and potentially unsafe applications.

Anti-virus and Anti-malware solutions with anti-exploitation features protect against zero-day malware, drive-by downloads, exploits and Exploit Kits.

Therefore, you need both an anti-virus and an effective anti-malware solution for maximum protection.

• The Difference Between Antivirus and Anti-Malware
• Malware Remover vs. Antivirus Software: What's the Difference?
• Antivirus and Antispyware Software: What's The Difference?
• What Is the Difference Between Antivirus & Antispyware?
• Use an Anti-Exploit Program to Help Protect Your PC From Zero-Day Attacks

If you choose to use Windows 8/10 Defender (or any other anti-virus), then I recommend adding an additional anti-malware solution. Please read Supplementing your Anti-Virus Program with Anti-Malware Tools for information about trustworthy and effective anti-malware programs which can find and remove adware, spyware and potentially unwanted programs.