I also have a little bit of (indirect) experience with software dev for ATMs, although it was in south east Asia. They were running Windows XP, and once you opened up the case you could just plug in a keyboard and mouse. From what I remember, don't want to be specific for obvious reasons, the screens in the ATM program are defined by a configuration file which is distributed from a central server. If you could intercept or modify that configuration, then you could get the atm to show whatever you want without having to reverse engineer the whole software.