ZeroAccess, or Sirefef, is a piece of malware known as a rootkit, this malicious peace of software replaces some critical files of the operating system and hooks to the kernel structure which makes the trojan invisible to both the operating system and security software.
Sirefef Cleaner
https://www.eset.com/me/download/utilities/detail/family/168/
https://kb.eset.com/library/ESET/KB%20Team%20Only/Malware/ServicesRepair.exe
https://kb.eset.com/esetkb/index?page=content&id=SOLN2895&locale=en_US
