I remembered there were issues with sspi and guest account. I searched and found this
kb
This is what you need to consider:
On a Windows XP computer that is a member of a workgroup: • If ForceGuest is enabled (set to 1), SSPI will always try to log on using the Guest account.
• If the Guest account is enabled, an SSPI logon will succeed as Guest for any user credentials.
• If the Guest account is disabled, an SSPI logon will fail even for valid credentials.
• If ForceGuest is disabled (set to 0), SSPI will log on as the specified user.
Additionally, if the Guest account is enabled, SSPI logon may succeed as Guest for user credentials that are not valid. The C sample code in this article demonstrates how you can check the
access token of the established security context. The IsGuest helper function in the sample code shows how you can verify that the logon occurred as the specified user or as Guest.