Hello Somedude,
Vista's parental controls should be enough to control what may be accessed on the computer, it does take some work in the sense that you need to have only one administrator account, (Yours
), and each of your children should have their own "Standard user" account.
all accounts should have passwords assigned. this should allow you to control access.
A more draconian method would be to physically disconnect the Router from the internet for the time that you wish to be internet free.
Also the router itself may have some form of access control built in , this is usually based on the unique identifier of the computer involved so is useful if each child has their own PC and may be adjusted to suit each child