Useful tool to scan USB drives for viruses on insertion

http://blog.didierstevens.com/programs/usbvirusscan/

Or try this one: http://experi3nc3.wordpress.com/2007/05/10/flash-disinfector-by-subs/

Download Flash Disinfector by sUBs and save it to your desktop.

• Double-click Flash_Disinfector.exe to run it. Follow any prompts that may appear.
• Your desktop will vanish for a while, and then reappear. This is normal.
• Wait until the program has finished scanning, then please exit the program.
• Restart your computer and see if problem still persists.