Changing the Spybot Check for updates task from System to my account,then Run only when user is logged on,and run with highest privileges stopped the Event 1903 from being logged in Event Viewer from 11:04 onwards.
Enabling the Check for updates task history after rebooting shows the task having Action started,Action completed when I logged on,and every 5 minutes thereafter.
Only problem is Spybot's Updates logfile isn't showing the check for updates every five minutes,so I guess that wasn't exactly a roaring success. :/
If you prefer not to have Event 1903 happening and would prefer to leave the Check for updates task disabled,that should prevent it.If you do go with that,you ought to make sure you remember to check for updates manually.
Or,if you prefer to reenable the check for updates,having Event 1903 happen doesn't look like it would cause any harm,so it's up to you.