According to a notice on the Microsoft Malware Protection Center, "Sality is a highly virulent strain […] known to infect other files (making full removal after infection challenging), copy itself to removable media, disable security, and then download other malware. It is also a very large family -- one of the most prevalent families this year." The fact that autorun can be used to start an attack process has helped this virus spread very fast.
The ~7MB patch just went live a few moments ago so it would be wise to check Windows Update to stay on the safe side. The update comes just over a week ahead of the regularly scheduled Patch Tuesday for the month of August.