From the product team directly: 

Every week seems like a big week for us in the engineering team working on our anti-malware technology. However, last week was especially important in a sentimental way. We got the final name for the cool technology our team has been developing for Windows. The name, after long consideration by our product marketing and branding folks, is "Windows Defender"! What's really cool about this name is that it’s more positive than "Windows AntiSpyware". Windows Defender is about what Windows will do for customers, defending them from spyware and other unwanted software. Our solution has really been about more than just the standard definition of "spyware". We’ve always said we will provide visibility and control, as well as protection, detection and removal from other potentially unwanted software, including rootkits, keystroke loggers and more.

Making the engineering change from "Windows AntiSpyware" to "Windows Defender" took a lot of careful coordination across our team to ensure that the strings in the UI got changed, the help files all got updated, registry keys, file names and properties, as well as a couple of images all got changed. All this work was completed and tested last Thursday, and is currently making its way through our build systems in Windows to make it into the main build environment, where official builds come from. We're pretty excited by the name, and by the sleek new UI and other improvements we've been making in it to help make Windows Vista the best operating system around! But Windows Defender is about a lot more than just a name change. The engine is now moved to a system service, and signatures are delivered over Windows Update. The detection mechanisms have also been radically improved by applying to spyware threats all the great detection technology we use in our antivirus engine.

As part of this engineering milestone, we've also started to prime the Windows Update software distribution channels with signatures for Windows Defender. This is important so the signatures are available when we ship the next beta. So, for enterprise and corporate customers that are using Windows Server Update Services, you will start seeing "Windows Defender" in the product category dialog as well as a new classification called "Definition Updates". So, now you'll know what that means.

In addition to the work that my team's been doing to develop this for Windows Vista over the past many months, it will also be available to existing Windows XP users, replacing the current Windows AntiSpyware technology we've been shipping in beta since January. More details on that in a future post. If you're not using our current Windows AntiSpyware beta, please give that a try!
I hope you like the name, and we can't wait to get Windows Defender into your hands to try. If you have any thoughts about the name, I'd be happy to read your feedback, and share it with out team. Hopefully in the next few days I can even get a screen shot posted for you!

RSS: http://blogs.technet.com/antimalware/rss.aspx

Urs