I don't know if this will do exactly what you want, but I thought I'd bring up some ideas for you.
You could install edubuntu or a similar small footprint linux base as the operating system and not give root/administration access to the guest users. You can really clamp down on guest privileges with a linux setup.
You could also use a standard windows installation with similar guest privileges coupled with net-nanny software to limit what guests are able to access (I assume you don't want guests looking at porn, gambling, etc. on your setup). A nice free net-nanny application I've used in the past is K9 web protect from Bluecoat, though you may need to purchase the pro version to legitimately use it in a business setting.
Group policy will do this, but read about it a bit before you use it. Improper use without understanding how user accounts and rights work can easily make the PC unusable. Using group policy you can setup which icons a user sees, which programs he can use, even block ability to right-click or select menu options.
Thanks a lot!!
While searching i also came across an app developed by Microsoft, called "Windows SteadyState", that also seems to do the trick!
thanks a lot for your help, ill try some things and come again here to tell the results!!