Windows Defender Beta2 expired on 31st December, please make sure you upgrade to Windows Defender Final.
Windows Defender is a free program that helps protect your computer against pop-ups, slow performance, and security threats caused by spyware and other unwanted software. It features Real-Time Protection, a monitoring system that recommends actions against spyware when it's detected and minimizes interruptions and helps you stay productive. Now with 2 free support incidents for Windows XP and Windows Server 2003.
Note: If you have version 1.1.1592.0, you must first manually uninstall Windows Defender before you can install this newer version. To configure or remove the existing version, use Add/Remove programs in the Control Panel. You can check your version of Windows Defender by clicking the down arrow next to the help icon and choosing ‘About Windows Defender’.