Since he's allowing guest users, that shouldn't be it. Ah! On the other hand, the guest account (by default) maps to a real machine account called "nobody". If you don't have a nobody account, this could indeed be the problem. Make sure you have a "nobody" account on your system. The alternative is to add the line

guest account = accountname

to your smb.conf file and make sure that the guest logins point to an existing account.