|is a software product from Microsoft to prevent, remove and quarantine spyware in Microsoft Windows. It is included and enabled by default in Windows Vista, and is available as a free download for Windows XP and Windows Server 2003.|
Windows Defender not only features scanning the system similar to other free products on the market, but also includes a number of real-time security agents that monitor several common areas of Windows for changes which may be caused by spyware. It also includes the ability to easily remove ActiveX applications that are installed. Also integrated is support for Microsoft's SpyNet network that allows users to report to Microsoft what they consider to be spyware, and what applications and device drivers they allow to be installed on their system.
I personally hate Windows Defender as it always caused problems for me. IT seemed to just be more of a pain in the butt by blocking legitimate actions I took, as opposed to a security feature.
As Derek points out, it will not technically conflict with AVG or ZoneALarm. The only reason you need to disable is if, like me, you are annoyed by it.