Unfortunately, the parental controls that come with Windows 7 are fairly basic, but it's still worth setting them up.
1. Click Start > Control Panel.
2. Locate User Accounts and Family Safety and choose Set up parental controls for any user.
3. Choose to set up parental controls for an existing user account or a new one.
On a shared computer, and especially one used by children, it's important for everyone to have their own user account and login. This offers greater control for you over what your children can do, it separates them from anything potentially unsuitable that's on your account and also prevents them from accidentally damaging anything of yours.
4. Under Windows 7 you can use the controls to limit the amount of time your kids may use the computer for. You set the times of day that they can sign in between and, outside of those parameters, they will automatically be logged off.
5. Parents are also given the ability to block selected programs as well as choosing whether to allow or block certain games, depending upon age ratings and type of content.
Find instructions for parental controls for other operating systems here.