Microsoft has released Windows SteadyState 2.5.
SteadyState is a free application that enables you to protect your PC and its contents from accidental or intentional harm.
It has been primarily designed for public access computers, for example in a library, Internet café, school or training centre. However it can able enable you to set up your Home PC, safe in the knowledge that the kids can't access or delete important files, accidentally remove programmes or change configurations.
Effectively it creates a space where the guest user can access the computer but can only access files and applications that you allow. In addition you can simply roll back any changes they do make ensuring the safety of your PC.
You can find out more about how SteadyState works in a variety of locations:
It's available as a free download.