Windows Defender expired last December 31, 2006.. ensure you have new version
Get the new version at Microsoft Website - http://www.microsoft.com/athome/security/spyware/software/default.mspx
Error that you might see is "Windows Defender application failed to initialize 9X800106ba"
Note: Windows Defender does not support Windows 2000 anymore. Beta of Windows Defender expired December 31, 2006
Solution is to update to newer version of Windows Defender.