The Antimalware Service Executable, also known as MsMpEng.exe, is a critical component of Windows Defender that runs in the background to protect your system from malware and other security threats.

I just noticed on my mom's computer that some "MsMpEng.exe" process (labeled "Antimalware Service Executable" and located under "C Program Files\Microsoft Security Client\") is consuming 100% of CPU.

This is an undesirable program. This file has been identified as a program that is undesirable to have running on your computer. This consists of programs that are misleading, harmful, or undesirable.