Rootkit

While security technologies become more sophisticate, malware writer also become cleverer to hide themself in the system. This kind of software and technique is called Rootkit.

What is a Rootkit ?
#
The term rootkit is used to describe the mechanisms and techniques whereby malware,
including viruses, spyware, and trojans, attempt to hide their presence from spyware blockers, antivirus, and system management utilities. There are several rootkit classifications depending on whether the malware survives reboot and whether it executes in user mode or kernel mode.
# (Quoted from SysInternals)

Why Rootkit is dangerous ?
Because it can do damage to your computer and yet go undetected by the system protection software.

What you should do?
There has been 2 Anti-Rootkit available which I know off.
RootkitRevealer from SystInternals

F-Secure Blacklight from F-secure

Download, install and scan your PC with it.