Many malware attacks triggered by USB devices:

One in every eight malware attacks occurs via a USB device, often targeting the Windows AutoRun function, according to Czech security vendor Avast Software. The company reported that of the 700,000 recorded attacks on computers in the Avast user community during the last week of October, 13.5% came via USB devices such as flash drives.

AutoRun alerts computer users when a new device is connected and helps them choose which application should run the new files. "AutoRun is a really useful tool, but it is also a way to spread more than two-thirds of current malware," said Avast virus analyst Jan Sirmer.

Avast urged users not to boot up PCs that already have USB devices attached, because the malware will load before some antivirus programs do. [Date: 6 December 2010; Source: http://www.computerworld.com/s/article/352998/]