Windows XP contains a guest account (turned off by default in XP Pro) that has the rather unique distinction of not having a password. Not only does the account not have a password, you can’t even set one. As you can see, if this account is active, it presents a slight security risk. Fortunately, there is an easy way to assign a password to the guest account.
To apply a Password to the guest account:
Using an account with administrative privileges,
Go to Start—>Run then type cmd press enter and enter the following command
Net user guest password
Now go to ‘start\control panel\user accounts’ and activate the guest account if it is not already. You will be able to assign and change its password.