Well, Jorge... logging on using the guest account isn't the best idea
anyway. Windows disables it by default... for a reason.

What exactly is the message you get?
On a XP service pack 2 machine, if I enable the guest account and try to log
in, I get a message saying "the local policy of this system does not permit
you to logon interactively".

Your alternatives are not just "administrator" or "guest"
I assume you're not using an Active Directory domain.
Try creating an ordinary user account on the server. Call the account
Jorge. Log onto the server using that account and see if you can use your
software.
Then create an account called Jorge on one of the user machines. Set its
password to be the same as the Jorge password on the server. See if you can
run your software from the workstation.

Jane