At times, the copyright protections imposed on products can make life difficult for legitimate users of digital content. In 2000, Microsoft Corp. introduced its online activation program, which required users to 'activate' their software, or it would not run. In 2006, Microsoft added its 'Genuine Advantage Notifications,' which check that a user's copy of Windows XP is legitimate. If not, the user is locked out of Microsoft's download sites, but can still receive critical security updates. The problem is that the 'Notifications' sometimes report users' machines as not having legitimate copies of XP when they actually do. In the music industry, the fact that CDs can be easily copied and duplicated by users has caused concerns. Copyright holders attempted to impose protections that proved to be either ineffective or very annoying to legitimate consumers. Digital music purchased online still comes with restrictions as to what can be done with it, however. Music purchased from Apple Computer's iTunes Music Store online can play on a maximum of five computers at one time, but if users want to play the music on non-Apple portable devices or personal computers running Linux, they must burn a CD and make a new version from that. This results in a loss of track information and reduces the quality of the audio. Software is available that allows users to strip restrictions from various digital content downloading sites. However, while this software is effective, it may come loaded with advertisements or spyware.