From Panda Security:
Panda USB Vaccine is a free utility from Panda Research which helps prevent malware infections due to the autorun feature of Windows Operating Systems. Once executed the user can choose two types of vaccinations: Computer Vaccination or USB Drive Vaccination. USB Vaccine allows users to vaccinate their PCs in order to disable autorun completely so that no program from any USB/CD/DVD drive (regardless of whether they have been previously vaccinated or not) can auto-execute. This is a really helpful feature as there is no user friendly and easy way of completely disabling autorun on a Windows PC. Panda USB Vaccine can be used on individual USB drives to disable its autorun.inf file in order to prevent malware infections from spreading automatically.