I have xp. I'm not sure if it is definition updates or program updates. What happened was yesterday there was a tiny little castle icon that popped up on the right hand side of my screen with a little x on it. So I clicked on that and it said Windows Defender needed to be updated so I clicked on the update button and it acted like it was going to update, but then it didn't and said something like Windows Defender is unable to update. I've never seen this type of update before so I'm not sure what kind it was. Usually defender updates li ke a windows download. I do have Windows Automatic updates set so that it asks first if I want to download any updates.