Malware definition data for mobile computing devices 2 is generated from master malware definition data 44 by selecting those classes of malware threat to which the mobile computing device is vulnerable and then selecting the matching malware items from within the master malware definition data. A PC 6 to which the mobile computing device may be connected is responsible for downloading an updated version of the master malware definition data for its own use and generates appropriate mobile computing device malware definition data for transfer to the mobile computing device when it is connected to the PC. The scanner programs of both the PC and the mobile computing device may be similarly updated.