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Can you ping the IP address of the 'unit' from the Samba server? I assume from your post's title that the 'unit' in question is a WindowsXP machine. Is it running XPHome or XPProfessional?
What 'distro' are you using for the Virtual private server? What version? Which kernel? What version of Samba?
Without more specific information, it is difficult to suggest the reason for your problem, however I would suggest that you look at the 'Linux - Networking' Forum. Then do a search of that Forum - possibly using your post title.
Saludos, y espero que tenga suerte!