The first step is to create an account for the child / person you want to limit access to.
Click your logon picture at the top right of the Start Menu and in the window that appears click the Manage another account link.
You will need to have a password on your [Administrator] account to set these up.
You now need to create a new user account for the person if you have not done so already. This will need repeating for each person you want set up in parental controls.
Make sure you set them as Standard Users and NOT Administrators.
Setup Parental Controls:
If you have already set up accounts, skip to Step 3. If you have already set up an account, or after you have done so, click on the relevant account and then click the link Set up Parental Controls.
Windows will now ask for what user you want to set up Parental Controls. Select the appropriate user.
You will now find yourself in the main controls window. Here you can select what controls you want to set up or modify.
In the time restrictions options, you can click and drag blocks to select when the user will be allowed to use the PC.
The parental controls will not let them log-in or use it outside of these times and will warn them when the end time is close. You can also choose if the user is allowed to play games and even, if you want, block specific games from being played.
The final screen allows you to choose a ratings system you want to use. Here, as this PC was set up in the UK, Windows has automatically chosen the BBFC rating system.
If you are unsure which to use, have a games. You can select the maximum age they are allowed to play to. This integrates with the Games Explorer from the Start Menu.