disable creating a new account in XP

Hi, My son asked me to use my computer and I let him use it after typing in the password logon (he does not know the password). The annoying thing is that he always created a new account with his own password. I don't want him to create any new account, so I wonder if there is any solution to prevent a new account from being created in Windows XP? In other words, how to disable the feature of "create a new account" in windows XP? Something to do with registry? cheers,

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