Open scheduled tasks in control panel--it is at the very tail end of System
and Maintenance.
You may need to turn on View, show hidden tasks in the right Actions column.
In the left tree view, you can expand Microsoft and see Windows Defender.
This should let you filter to the right event log entries. However, I'm not
sure what you see will help.
Choose the task in the top center window, then the history tab in the lower
window.
If you double-click a given record--say, a task completed record, you should
have a copy button, and get something like what's below--lets see if it
gives you or the rest of us any useful clues.
I've removed my computer and usernames before posting the log record--you
may want to do that--or just post any relevant codes for the failure.
I also left out the XML, to save space, and you will note that my record is
not from Defender--but what you see should be similar.
---------------------
Log Name: Microsoft-Windows-TaskScheduler/Operational
Source: Microsoft-Windows-TaskScheduler
Date: 3/18/2008 2:28:00 AM
Event ID: 102
Task Category: Task completed
Level: Information
Keywords:
User: SYSTEM
Computer: removed
Description:
Task Scheduler successfully finished
"{83384e01-7c8c-447c-b16a-c1fdb8edee69}" instance of the
"\Microsoft\Microsoft Forefront\Client Security\Client\Antimalware\MP
Scheduled Scan" task for user "removed".