Microsoft has released the emergency, out-of-cycle security patch I talked about last week. What's interesting is that it seems that it works even on Windows XP/Service Pack 2. Microsoft ended support for XP/SP2, but it seems the patch is good on it.
This is nasty, and is being widely exploited, so wander on over to Microsoft for the security goodness. You'll need to reboot when you're done.
UPDATE 4 August 2010 10:11: If you run Windows Update and your computer rebooted last night, you almost certainly have the patch. If you don't run Windows Update, you should.