could be the windows machine misreporting. Disconnect the network cable, reboot and try again?
Depends what the IP Address lease times are too. if they are 24hrs then they won't be given up until AT LEAST 12 hrs. You could try re-installing the IP Stack on XP?
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http://www.online-tech-tips.com/comp...lready-in-use/
See link (point 4 may be relevant)