CWmike writes "Microsoft has started adding Security Essentials to the optional download list seen by US Windows users when they fire up the operating system's update service, and antivirus rivals are crying foul. 'Commercializing Windows Update to distribute other software applications raises significant questions about unfair competition,' Carol Carpenter, a GM at Trend Micro, said on Thursday. 'Windows Update is a de facto extension of Windows, so to begin delivering software tied to updates has us concerned,' she added. 'Windows Update is not a choice for users, and we believe it should not be used this way.' If Windows doesn't detect working security software on the PC, Microsoft adds Security Essentials to the Optional section of Microsoft Update, a superset of the better-known Windows Update, or to Windows Update if it has been configured to also draw downloads from Microsoft Update. Microsoft made a point to say that it was not offering the software via Windows Update, but only through the Microsoft Update service, which also offers patches for new versions of non-operating system software, notably Office and Windows Media Player. But most users won't understand the distinction."