Vista - Windows Defender scheduling problem - scratching my head

Hope someone can help...

Dell M1330 notebook with Vista Home Premium SP1.

Problem with Defender is as follows....

Every 3 days or so, if a manual scan has not been run, I get the icon reminding me that a scan has not been run for several days. Fine so far...

So I look at the WD tools | options | scheduling info and it's set up to run at 20:00 each evening as a quick scan. Fine so far... the notebook is on at that time usually.

Investigate the hidden Windows Defender 'MP Scheduled Scan' in Task Scheduler...

So that's it! The power conditions are set for the task to run only if the computer is on AC power... well I'll just uncheck that box then so that's not an issue anymore. While I'm at it, I'll also go into the settings tab and ensure the task runs as soon as possible after a scheduled start is missed.

So I make the Task Scheduler changes, OK them, exit... I can go back in and check and everything now looks OK....

But it doesn't run! And you know why? In the history tab of the MP Scheduled Scan task, it appears that each time, a couple of mins after the notebook is booted up, User 'NT AUTHORITY\System" deletes the WD scheduled task, and a second later updates (or recreates) the scheduled task again with the 'AC power on' check back in, and removes the 'run task as soon as possible after a start is missed' check from the option!

Just to add that I've disabled scheduling from within WD, deleted the Task Scheduler job and then re-enabled automated scans in WD (ensuring that the TS task is recreated) but the same sequence of events re-occurs.

Can anybody please suggest why this is happening, and what I could do to fix it?

TIA,

Mike

NT AUTHORITY\SYSTEM is what schedules the MpCmd.exe (defender)
If it changes your scan options, It may no longer recognize your User account in the security database as authorized for changing the Task Scheduler.
This may be a result of User file corruption. If you have been using registry cleaners it may have resulted in Windows/User File corruption. Try a system restore to a point before the problem occured (if said restore point still exists), or attempt a repair of Vista and/or reinstall Vista. Stop using registry cleaners. Click here: Run an antivirus scan

"MP Scheduled Scan.job" (MpCmdRun.exe) ** ERROR **
The attempt to retrieve account information for the specified task failed;
therefore, the task did not run. Either an error occurred, or no account
information existed for the task.
The specific error is:
0x8004130f: No account information could be found in the Task Scheduler
security database for the task indicated