In Windows XP Professional by default the guest account is disabled.If the Guest account enabled you can set it to disabled,as it is recommended to disable the guest account in a domain or to computers that do not use simple file sharing. For security reasons to prevent attack of hackers guest account should be disabled .
Start->settings>Control Panel>Administrative Tools
Double click Computer Management ,in the left pane select Local users and Groups>Users and in the right pane ,
Select Guest and right click Properties and in General tab enable or check Account is disabled.