Is your computer filled with files that aren't yours? Desktop cluttered with downloads that you didn't download? Do you have spyware even though you know all the steps to take to avoid getting it? It sounds like you might be sharing a computer.
Whether your shared computer experience is at home with your family or you're an administrator of shared computers in a school or library or other public place, consider downloading Windows SteadyState. Windows SteadyState can return your shared computer and hard disk to the condition it was before anyone changed it. You just restart your computer.
There's no charge and Windows SteadyState now supports Windows Vista.
For more information, see Shared Access Computing with Windows SteadyState.