Troubleshooting Windows Defender
Windows Defender is a new security program that helps to protect your PC from spyware and other unwanted software. This program works on Windows XP & Vista. You may encounter various issues with the Microsoft Windows Defender application. This article explains you the various issues with the troubleshooting procedures.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB555957.