Answer:
Well, it's essentially the old Windows Defender. Microsoft dropped it in Windows 8, so you can't use it in any Windows version past that.
Step-by-step explanation:
The basic premise is that it would scan for malicious software like malware, spyware, trojan horses, ransomware and rootkits (which are pieces of software which grants the creator of the software access to your computer). It would monitor activities and constantly check for files that may be malicious - again, similar to Windows Defender.
It was introduced in Windows XP and dropped in Windows 8 - replaced by Windows Defender (I'll stop going on about that now.) - which does the same thing, just with updated databases and it's faster (sometimes).