I'm not familar with a Window's "guard." Why not set up a seperate account for them and password protect
your logon as Admin?
From
Microsoft's Help Desk...
To add a new user to the computer
You must have a computer administrator account to add a new user to the computer.
1. Open User Accounts in Control Panel.
2. Under Pick a task, click Create a new account.
3. Type a name for the new user account, and click Next.
4. Click either Computer administrator or Limited, depending on the type of account you want to assign to the new user, and then click Create Account.
If you see an account named Owner, rename that account with a user's name. The Owner account, with computer administrator privileges, is created during installation if no user accounts are set up at that time.
Of course, you'll want to put give the kids LIMITED access. This will prevent them from being able to access any of your files, folders, etc.
Here's another link that might help...
Types of user accounts