Windows vista has some nice parental controls. I have always thought that the Macintosh had nice controls for restricting a child's ability to run certain software, and visit certain sites on the internet built in, and I always wondered why they didn't promote that more... now it's too late beacause that's no longer an advantage they hold over MS.
Vista allows an administrator to further lock down a regular user account so that that user can only access programs from a specified list, or only games with a certain rating (and you can pick which rating system you prefer from a list of several.) or only access the internet between the hours of X and Y. you get the idea. It's cool and it's free the only catch is it gets turned off if the PC is being run in a domain environment (corporate installs) . I think it would have been handy to have there too. I would love to be able to restrict access like that on some of the Point of Sale machines I work on, maybe it'll happen when Longhorn server comes out and Active Directory gets updated.