Perhaps it behaves differently depending on which OS is in use. On my Vista box,
if you double clicked the Defender icon in control panel, it said "defender is stopped", or words to that effect.
In that situation there were no updates to Defender, not until I restarted it, many months later.
Defender can be set so that real time activity is inactive but it still does a scan once a week, and still updates
while it is in this state.