When I try to directly access either XP machine from the Linux machine, I receive the message "Connection refused."
Also, this chapter from the O'Reilly Samba book is pretty good.
The key thing that the article gives you is the method for finding out more information from the logs on the Linux server. It might give you a more detailed error message as to why there is a failure. It could be as simple as the guest account on the Windows XP PC is not set up correctly, but it's hard to tell without some better troubleshooting information.
Maybe there are hints on the Windows machine as to why the account is failing. You could use the Windows Event Viewer to see if the failures are registering there and if there are any kind of error codes that might be helpful.
This was first published in December 2004