XP-Professional already has a guest account, so all you have to do is enable it.
From that ControlPanel/UserAccounts:
Click Advanced Tab
Click Advanced Button
Click Users in left panel.
In the right panel you should then see the "Guest" account with a red-X indicating it is disabled.
Right-click Guest Account and choose "Properties".
Un-check "Account is disabled".
>> Stay informed about: Unable to add either New or Guest account