In an interview Tuesday, Jerry Bryant, a group manager with the Microsoft Security Response Center (MSRC), said that the decrease in infection attempts -- ones stymied by a Microsoft antivirus signature -- was due the AutoRun update preventing large numbers of primary infections.
"We attribute the overall decline in infections to fewer systems trying to propagate using AutoRun," said Bryant.
In February, Microsoft noted that the AutoRun update would break the functionality of some USB drives. "Users who install this update will no longer receive a setup message that prompts them to install programs that are delivered by USB flash drives. Users will have to manually install the software," Microsoft warned in a security advisory at the time.
The company has also published the "Enable Autorun" tool that customers can deploy to disable the update's changes and revert to Windows XP's and Vista's earlier behavior.