Threat Spotlight: Trojan.
Trojan.Zeroaccess is a Trojan horse that uses an advanced rootkit to hide itself. It is often installed through drive-by-download attacks from sites hosting the Blackhole exploit kit. The Trojan can also create an encrypted, hidden file system, download more malware, and open a back door on the compromised computer.
The Trojan is called ZeroAccess due to a string found in the kernel driver code that is pointing to the original project folder called ZeroAccess. It is also known as max++ as it creates a new kernel device object called __max++>.
More information on Trojan.Zeroaccess is available in the threat family writeup.