I've been asked many times about parental controls that are part of the Windows operating system.
Windows XP: There are some parental controls built into Windows XP but they are limited. There is some website filtering and program control but it is by no means fool proof. Here is a link to a decent overview of parental controls in Windows XP:
Windows Vista: Though I HATE Vista, Microsoft did improve the parental controls included in the operating system. The parental controls built into Windows Vista help parents determine which games their children can play, which programs they can use, and which websites they can visit—and when. One of the coolest features is the ability to restrict computer use, by user, to certain times of day. So if you don't want your kids online while you are away at work or after 9pm, you could enforce such rules. Here is a link to an overview of parental controls in Windows Vista:
Windows 7: I just upgraded to Windows 7 and am really liking it so far. The parental controls in Windows 7 are pretty similar to Vista with a few improvements. Here is a link to an overview of parental controls in Windows 7. When you go to the page, the real "meat" of the overview is in a 38 minute video in the top left portion of the screen: