If your Windows Defender does not update or if you ever encounter an error code (eg "error code 0x80240029 - cannot update" which btw is the most common one), when updating Defender, here is a solution which will help you. This usually happens if the distribution database is broken and needs to be re-created.
Here are the solutions to address the issue :
1) As far as Vista goes (don't know if this applies to XP) you can do this by clicking start > all programs > windows updates> change settings > uncheck Automatic Updates, Uncheck Recommended Updates and uncheck use Microsoft Update Service. this will switch from Microsoft Update back to using Windows Update.
Reboot. If it works for you fine; you can now reset the update settings to as they were earlier.
2) I also came across this download from Microsoft (Vista/XP). This actually FORCES defender to update !
To manually download the latest definition updates for Windows Defender:
Save it to your desktop. 'Run As Administrator' this mpas-fe.exe
When the definition file runs, a file extraction dialog box appears. After the file extraction dialog box closes, verify that the Windows Defender definitions have been updated.
4) Lastly if none work, there is another way you can try this:
Boot into safe mode. Find the folder named Software Distribution.
Rename SoftwareDistribution to SoftwareDistribution.old OR Delete all its contents (FLUSH it !)
Reboot. Now try to update the Defender (at this point a new SoftwareDistribution folder will be created if you have renamed it).
