This post is marked as Deprecated and therefore contains outdated information.
Rootkit Viruses are stealthy viruses that can cause great damage to your operating system and even to your hardware if they are in the “firmware rootkit” class. Several classes of rootkit viruses exist: persistent, memory-based, user-mode, kernel-mode and firmware driven. Installation of these rootkit viruses are automated and can evade many anti-virus programs. Removal of these viruses can be difficult, especially if they are the kernel-mode or firmware driven versions.
The latest rootkit virus that seems to be causing much damage and is spreading at a medium rate is the Rootkit.Sirefef.Gen.
There is a rootkit scan tool called RootkitRevealer v1.71 from microsoft support.
There is also a rootkit remover tool from the bitdefender website available.