He's has the Guest account disable over the network. That's why Justo/guest is the local account on Justo's computer. XP uses the Guest account by default for file sharing. So that's what your computer is trying to do is login using the Guest account on Justo's computer.
You both need to disable the Guest account in Computer management for 2000 Pro and XP Pro, XP Home users can't, they need to execute a command.
Open up command prompt, type net user guest
If it says Account active Yes
You want to disable it with the command net user guest /active:no