This is a common requirement for computers that are accessible unsupervised by the public. Such examples are kiosk computers, self service tills etc.
In a previous job I worked on developing a weighing scale for a supermarket. The weighing scale was used by shoppers to weigh their own fruit and veg and print price labels. The device was basically a PC running Windows XP Embedded and had USB ports underneath, making it possible to plug a USB keyboard in and if you knew what you were doing, you could get into the OS (necessary for maintenancy).
To prevent malicious use/attack the device use a feature of the embedded OS called the Enhanced Write Filter. This feature uses a RAM disk as a layer between the physical disk. All disk writes are held in the RAM disk; the hard disk doesn't see them. When the device is rebooted or loses power the RAM disk is lost along with any writes it held. This meant that if anyone managed to attack the device (eg deface it with something nasty, eg porn) all the store staff had to do was switch it off and on again.
For maintenance purposes you can commit the RAM disk changes to the hard disk but this requires an admin password.
XP Embedded is pretty old now, but I believe the feature is also included in Win 7 Embedded.