The Guest account and any limited accounts do not have the ability to
create any Windows user accounts. This is a task that is only available to
accounts in the administrator group.

If the new accounts are showing up in Control Panel> User Accounts, it is
possible that the "guests" are signing in with the hidden administrative
account. Under certain conditions, that account does not have a password
until one is assigned to it. If this is the case, add a password to the
account so that others cannot access it.

XP Pro: Press CTRL+ALT +DEL twice while at the Welcome Screen. When the
classic logon prompt appears, type in Administrator as the user name. Leave
the password field blank and press enter or click the green arrow. Once
logged on, go to Control Panel> User Accounts and select the Administrator
account. Then choose the option to add a password.

XP Home: Boot to Safe Mode. Select the account named Administrator. And
proceed to assign a password to the account using Control Panel> User
Accounts.

If on the other hand, you have ".NET Passport" accounts being added to your
system's configuration, that's a different matter. This kind of information
shouldn't be stored in the normal Guest account but log on with the account
where they're showing up. Then go to Control Panel> Users. Select Guest (or
whichever account is showing the new account names) and then click the
"manage my network passwords" in the left task pane or select "change my
.NET Passport" in the "what do you want to do" area (main area of page). On
the next screens you can pick and delete unwanted passport accounts that
may have accumulated.

The only other kind of account that I can think of that might be showing up
would be ones associated with your ISP. Those are not managed by Windows at
all. Check the ISP site (or with their tech support) for info on how to
control your account so that new accounts associated with your main login
cannot be created.