http://www.microsoft.com/windows/products/winfamily/defender/support.mspx And, Windows Vista is an operating system made by Microsoft which contains a built in malware protection tool called Windows Defender, which can also be downloaded and run as a stand alone service for Windows XP. Windows Defender is a part of the Vista operating system, so it cannot be removed without risking total failure of the operating system. Hence, for Vista users, Defender should be disabled if it's services are not needed. In Windows XP, the stand alone install of Defender can be uninstalled normally. http://www.ehow.com/how_4881875_uninstall-windows-defender-manually.html

"Unless I totally misunderstood my readings, AVs and malware detection applications are niche services. " I think this depends on the particular software, the intended message and context of the conversation. Just as an example (not intended as proselytizing), I use what I think of as Norton Anti Virus. But it's really the Norton Internet Security system. It's a suite of many, many programs and routines that handle a large number of functions. Website reputation, antivirus-malware-spyware-phishing blocking and removal, firewall, temp file cleanup, safety of files downloaded from websites or emails, email scanning, and on and on. Other than the occasional message that pops up, it's invisible. AND, it schedules scans on the fly for only when the PC is not in use. It never interrupts or delays other activity. I never touch it, never schedule anything, and almost never see anything. I think all the best in class products (there are maybe 4 or 5 best choices) are similar to what I described, comprehensive "systems". They're well worth the modest annual cost.

I cannot find much about this error except that it does appear to be some update issue, at this time I would suggest visiting the windows update page and see if there is possible up date for Defender that's all I can offer for now and will look around to see what else or if anyone has had the problem and a solution to it. zep

Not on windows 8.1. Defender on windows 8.1 is basically Microsoft Security Essentials, if you were to install avast Defender I think is automatically disabled.