That's because Microsoft has surreptitiously released an anti-hack detection update via Windows Update. Those who chose to have Windows Vista install updates automatically would have this update installed and running by now.
"You can see that the KB940510 update is listed as Important. At this point, most users would assume it's an important patch that fixes flaws in Windows Vista and install it. Double-click on the update and you will find this short description."
Read more at TechARP.