Some other program like chkdsk is currently running which locked the
disk and so BITS is not able to write to the disk.
If the issue is not being caused by a scheduled check disk, then
"something" else is locking the disk or preventing BITS from writing to
the download location.
That "something" could be security software [antivirus/3rd party
firewall/etc], malware, or the Hard Drive may have issues.
If this is Windows Vista, open the Services console. *Right* click
Computer either on the Desktop or Start Menu and choose 'Manage';
allow the User Prompt; open Services and Applications; then Services.
click on Background Intelligent Transfer Service and see if you are able
to Start it.
Then try to update Defender again.
Alternatively, you can manually download the latest Defender definition
from here, save it, and then manually install it:
> Definition Update for Windows Defender - KB915597 (Definition 1.63.1021.0)
> I Get Error Code 8020000D When trying to install update.
> How do I fix it?
> Thanks Hooter