Windows Vista Security Center Black Death

Running Windows Vista? Ever seen anything like this? Then you were probably as frustrated I am right now. For some odd reason, one of my computers one morning this week alerted me to the fact that it was lacking anti-spyware protection. I hadn't changed any settings, and Windows Defender was still installed. I don't run anti-virus software on this computer (well, any of my computers), so the "Not Monitored" setting is normal. What's strange about this problem is that clicking Update Now to update Windows Defender results in...nothing. I'll get a User Account Control prompt, but nothing else will happen. Windows Defender will still do a scan, but it has the yellow exclamation mark on the system tray icon telling me it's out of date. This seems like it's related to some sort of broken Windows Update functionality, but the manifestation of the black sidebar is the curious part.

When this problem happened I did a System Restore, going back as far as I could (which was seven days - anyone else noticed that Vista's System Restore doesn't go back as far as XP did?) and, bizarrely, it didn't fix the problem. That's unusual because the problem happened a few days ago, so the earlier System Restore points should have done the trick. I had this problem happen months ago, and System Restore fixed it - I'm not so fortunate this time. I've done a fair bit of Google searching and I can't find anyone else referencing it - though that could be a matter of syntax on my part. Every search I do that includes the word "black" gets me "black hat" security results.

I'm stumped. Any ideas for me to try?