It sounds like you need to configure your Univeral Plug and Play controller in Windows XP to give out the same address each time. I don't know if this is possible (and being on win2k, I can't investigate myself), but the microsoft web-site would be a good starting point (see http://www.microsoft.com/windowsxp/expertzone/columns/crawford/02july22.asp for starters).

Cheers
Paul