Microsoft appears to have fixed an error in distributing Windows Defender definition updates on Windows 7. The problem started manifesting around June 11, when German blogger G..nter Born noticed the problem. Windows Defender is offered as a free download for Windows 7 users, and it’s only an anti-spyware solution. The older OS is not the default security software, but it’s not the standard security software on Windows 8 and is only available for free download from Microsoft’s site, suggesting this was only a temporary bug.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-resumes-delivering-windows-7-defender-definition-updates-after-2-weeks/