Windows Defender vs Security Essentials on W7 Pro

Microsoft has confused things by using the same name to refer to different programs.

For Windows 7, Microsoft Security Essentials and Windows Defender are different types of applications.

MSE is an antivirus program.  Defender is an antispyware program.  

Wikipedia:

Windows Defender, officially called Windows Defender Antivirus in Windows 10 Creators Update, is an anti-malware component of Microsoft Windows.^[2]^It was first released as a free antispyware program download for Windows XP, shipped with Windows Vista and Windows 7 and made into a full antivirus program replacing Microsoft Security Essentials as part of Windows 8 and later versions.^[2]^

Wikipedia:

Microsoft Security Essentials (MSE) is an antivirus software (AV) product that provides protection against different types of malicious software such as computer viruses, spyware, rootkits and Trojan horses. Before version 4.5, MSE ran on Windows XP, Windows Vista and Windows 7, but not on Windows 8 and later, which have built-in AV components.

If all you have installed is Defender (which is installed in Windows 7 by default), you're not sufficiently protected against malware.  You need an antivirus program, whether it is MSE or something else.  Defender can be used with whatever antivirus program you select, although it may be redundant with some.

MSE disables Defender in Windows 7; or should.

https://answers.microsoft.com/en-us/protect/forum/protect_start/windows-defender-and-microsoft-security-essentials/5309cb8d-02e1-40e8-974f-0dcedb9ab9fd

Q: Is Microsoft Security Essentials (XP/Vista/7)designed to replace Windows Defender?

A: No but if you are running Microsoft Security Essentials, you do not need to run Windows Defender. Microsoft Security Essentials is designed to disable Windows Defender in order to manage the PC’s real-time protection, including anti-virus, rootkits, Trojans and spyware.

**Q: Does installing Microsoft Security Essentials (XP/Vista/7) disable Windows Defender?A:**Microsoft Security Essentials should disable Windows Defender on Vista and Windows 7 and uninstall it from XP. In some cases, this does not happen automatically.

**Q: What happens if I do not disable or uninstall Windows Defender if I am using Microsoft Security Essentials (XP/Vista/7)?**A: If Microsoft Security Essentials and Windows Defender are both running, your system may experience performance degradation and other problems caused by the conflict of two services providing real time protection simultaneously.

Cheers.