It is possible to turn on parental controls if your machine is on a domain, but default it is disabled. It is not possible to turn on parental controls for Business skews of Vista since parental controls is not present on the machine at all in these skews of the operating system. If you are using Windows Ultimate and have your machine joined to a domain you can enable parental controls by using a group policy setting. Even though it is posssible to turn on parental controls on a domain, it is only possible to apply parental controls to non-domain users. So you can only turn on parental controls for local machine users and not users on the domain.
To edit the group policy settings you go into the control panel and into the Administrative Tools section to the 'Edit Group Policy' control panel. This will require elevation and you need to be elevated to do this. Inside the Group Policy Object Editor, you go to Computer Configuration/Windows Settings/Windows Components/Parental Controls. Inside this section there is a group policy option called 'Make Parental Controls control panel visible on a Domain'. Enable this option and you will be able to get the Windows Parental Controls control panel and turn on parental controls for users on the machine.