Trojan:Win64/Sirefef.AC is a component of Win64/Sirefef - a multi-component family of malware that uses stealth to hide its presence on your computer. The family consists of multiple parts that perform different functions, such as downloading updates and additional components, hiding existing components, or performing the payload.
Caution: Win64/Sirefef is a dangerous threat that uses advanced stealth techniques in order to hinder its detection and removal. As a consequence of being infected with this threat, you may need to repair and reconfigure some Windows security features. Please see Additional remediation steps in this entry for more information.