Cannot Update Windows Defender !?
If you ever encounter an error code (e.g. "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 !
3) To manually update Windows Defender visit Microsofts Malware Protection Center for instructions.
4) Lastly if none work, there is another way you can try this:
Boot into safe mode. Find the folder named Software Distribution.
Rename SoftwareDistributio n to SoftwareDistributio n.old OR Delete all its contents (FLUSH it !) Reboot. Now try to update the Defender (at this point a new SoftwareDistributio n folder will be created if you have renamed it).
Additional help can always be found at Microsofts Windows Defender HelpGroup .