Always make sure children use the computer in an area where you can easily keep an eye on them.
Use a timer
Use time-based rules to prevent Internet use (or computer use) during off hours. It's built into Vista's User Accounts (even generates user-activČity reports for you), and also in most parental-control software. You may even find a setting in your network router.
You don't give the kids carte blanche on the PC, so don't extend it to game consoles, either. Xbox 360 and NinČtendo Wii can limit kids' play based on ESRB ratings for games. Xbox also sets limits based on MPAA ratings
for DVDs. PlayStation 3 has different restriction levels. (Reset that default password of 0000, or Junior will reset it for you.) Xbox Live now includes
a family timer, to prevent too much gaming per day or per week.
Beware new software
Kids find out fast if a parental control is limited to a certain kind of browser or software and will just download something new. Buy controls that work systemwide. Net Nanny Home Suite (Net Nanny Parental Controls from ContentWatch) is the most mature parental-control software available.