There are several Kiosk type softwares (every time it boots, is exactly the same as previous time, all modifications rolled back each time [unless made permanent by some means]).
M$ has one called
SteadyState [freeware, XP32 amd Vista - Not W7+] (
https://en.wikipedia.org/wiki/Windows_SteadyState), I have it but never as yet tried it.
At least one third party alternative
Deep Freeze [trialware] (
https://en.wikipedia.org/wiki/Deep_Freeze_(software) ), I'm sure that there are probably other alternatives.
A decent Firewall is always a pretty good idea, even if you dont use an anti-virus.
EDIT: SteadyState is no longer available, since 2010, but probably available somewhere, (I still got it).
Also,
Reboot Restore Rx [freeware]:-
https://en.wikipedia.org/wiki/Reboot_Restore_Rx
and
ToolWiz Time Machine :-
http://www.toolwiz.com/lead/toolwiz_time_freeze/