The worm checks all the drives on your PC until it finds a mapped drive. It then tries to copy itself to that drive, and creates an autorun configuration file named autorun.inf.
Autorun configuration files are used by many different software and are designed to automatically launch or open a program whenever you access the drive that has the file. This is most often used on CDs or DVDs for installing software.
In this case, the file is used to automatically run the worm on your PC.
Alerts from your security software may be the only symptom.