If you purchase a music CD from Sony and insert it in you computer, it may install a rootkit (user-level or kernel-level software designed to hide itself and its related files and registry keys--a common malware practice) which cloaks Sony's proprietary DRM software designed to limit copying of the music on the cd.
Sony's End User License Agreement does not mention the rootkit. Furthermore, no method is provided to uninstall it. Attempting to manually remove the rootkit can leave your cd-rom drive disabled, in fact, since the Sony software installs drivers that filter i/o to the cd-rom driver and the ide channel: a "lower filter" driver called $sys$crater to cover imapi, and an "upper filter" driver called $sys$cor.
$sys$crater and $sys$cor are hidden. Infact, the rootkit hides any files, folders, registry keys, and processes which begin with "$sys$". A quick way to find out if you have been "trick or treated" by Sony is to go to Device Manager and under the Details tab for your CD-ROM drive select "Device Lower Filters". If $sys$crater shows up it is indeed halloween. (Another way is to rename something so it starts with "$sys$" and watch it disappear.)
Mark Russinovich of SysInternals.com discovered this, and posted the details.
But the weirdest thing is that Sony is not even seriously trying to prevent illegal copying with their CD DRM scheme. Instead their target is Apple. At this point the story gets even more prankish and halloweenish, which you can read about here:
DRM Crippled CD: A bizarre tale in 4 parts