Asking you what action to take is not the same thing as taking an action without your knowledge. The computer recognizes that new hardware was installed and wants to know what to do with it. It doesn't read anything from the new hardware until you tell it to.
I suggested a new user with restricted access because even if it corrupts that user, it can't corrupt the system. You can just delete the user and eliminate anything on the usb that got moved to your computer.
Create user with restricted access. Make sure that user has access to your updated virus software. Sign on with that user. Put in USB. Scan the USB for viruses. Done.