Sony's DRM software puts a hidden 'rootkit' code on the user's PC when the CD is inserted into the PC. This code can be used by to conceal any kind of file without restriction making it an ideal hiding place for viruses and other malicious code. Needless to say several Trojan horses quickly appeared.
An announcement by Microsoft (not Sony) that it would provide software to remove the offending code appears to have been the last straw for Sony, which said that it was suspending production of the CDs. Sony offered to replace the 2.1 million discs that had been sold and withdraw a further 2.6 million from store shelves.