Well, seeing as it's theoretically a "trusted environment" you could go for security through obscurity. Ever wanted to learn the dvorak keyboard layout? Perhaps a keyboard with no labels at all would be helpful. Or perhaps a KVM switch hidden behind the computer, without the monitor going through it. Press the key combination before you leave, and they can type all they want but it won't go through to your computer.

All of this is easily bypassed if the person cares enough, so it's really only good if there are other computers around they can use instead. Most solutions are going to work with windows' login process, so if you have limited rights on the computer it's going to be hard to find a solution that works for you. Are you sure you don't have the ability to modify the permissions on your own files?

Matthew