Keep separate accounts on your computer for you and your kids!
The main point here is that a kid that has administrator privileges on a computer will be able to install and uninstall whatever they want. When they see a "neat" online game, they can just download and run it without any concern for whether it's infested with spyware or viruses.
By giving them only a "limited" user account, any install of a program or major change to the security settings of the machine will require your input thus giving you a chance to exert your parental influence. For example, you might not want your 8 year old to try the demo version of Manhunt.
Besides the security issues, having separate user logins for each member of the family makes it possible for each person to have their own background picture, sound effects, and icons. This is a lifesaver for the parent who just doesn't want a Star Wars or My Little Pony background picture and can't ever seem to find their document among the massive desktop shortcuts to games and chat sites.