Go to the Start menu; it's where the MS logo appears. Just left click on the Start. A lock will appear. The computer can be locked.

For the function to work, though, one must first use the Control Panel and set up a primary user-account. Then, to unlock the computer, a password-prompt appears.

I'm on Vista, and there is no cancel button or option that displays once the computer has been set to "lock" mode - only the options to input the primary password or switch user. (If children have the primary password, then, maybe it's time to change it.)

As someone else mentioned, it's through the Control Panel where the computer's administrative account or primary can set limits on what other users of a shared computer can access.

P.S.: When the Control Panel, Parental Controls are accessed, click on "Why do I need an account for this?" and a window opens that offers in-depth info. about the feature, i.e.:

You can use Parental Controls to help manage how your children use the computer. For example, you can set limits on your children's access to the web, the hours that they can log on to the computer, and which games they can play and programs they can run.

When Parental Controls blocks access to a webpage or game, a notification is displayed that the webpage or program has been blocked. Your child can click a link in the notification to request permission for access to that webpage or program. You can allow access by entering your account information.

Before you get started, make sure that each child that you want to set up Parental Controls for has a standard user account because Parental Controls can only be applied to standard user accounts. To set up Parental Controls for your child, you'll need an Administrator user account. Parental Controls cannot be applied to an Administrator user account. For more information about user accounts and setting them up....