A Windows Defender 0x80240029 error says that “Windows Defender cannot download updates”. Here is a solution that may work for you:
1. Open Windows Updates by going to Start | All Programs | Windows Updates
2. In the left hand pane click on “Change Settings”.
3. Turn off Automatic Updates and uncheck “Check for Updates”.
4. Now, click “Check for Updates”. It should find some updates, as well as the Windows Defender update.