I am not sure if this will help or not. I use a "sandbox" program called Toolwiz Time Freeze. It can be configured to automatically run when Windows 7 starts. It prohibits any and all changes to your computer (including any web links). When you turn off the computer all "changes" are discarded.
It also has the ability to be password protected. Meaning in your example, no one using your computers will be able to save or download anything (including PUP's, viruses, malware, etc.). You, on the other hand, will be able to turn Time Freeze off (so you can make changes, downloads, etc.) since only you know the password to turn it off. If you set it to launch with Windows, it should be almost full-proof protection, if I understand correctly...