Windows Defender runs very silently (unless there's a detection), no system tray icon or notifications of updates. To view Windows Defender, or run a scan go to the Start screen, type Windows Defender, and the Settings filter.
You cannot uninstall Windows Defender, but if you install another anti-virus program Windows Defender will disable itself. Windows 8 Consumer Preview, and Release Preview will allow Microsoft Security Essentials to be installed. If you've installed Microsoft Security Essentials, uninstall it now.
More info on the improvements to Windows Defender: http://blogs.msdn.co...om-malware.aspx