There's not a big difference between rootkits and syware, and Microsoft has warned users early this year that this is likely to be the successor of spyware. Rootkits are almost impossible to detect using current security products and could pose a serious risk to corporations and individuals.
These harmful elements run quietly in the background and can be spotted by looking for memory processes that are running on the infected system.

The malicious spying programs are becoming more common and could soon be used to create a new generation of mass-distributed spyware and worms.

Such a tool for snooping on users isn't used just by hackers; as it turns out, some major music publishers are also taking an interest in this feature. One such rootkit was discovered by Mark Russinovich on one Sony music CD he owned. Details about this discovery have been posted in the Sysinternals blog.

The rootkit installs in the Windows system and is basically telling the OS what to do and it won't take "no" for an answer. This way, files inside the rootkit are invisible from within Windows.

Once a CD protected by Sony's DRM is played on a PC, an End User License Agreement pops on screen, which defines the terms of use of the CD and must be accepted. However, it doesn't say anything about the existence of the rootkit and the installation of this code which will take place without the user ever knowing about it.
Trying to remove the rootkit will lead to further problems, such as discovering that the CD drive is missing from Explorer. Deleting the drivers will disable the CD.