On the XP machine, in windows explorer. Tools > Options > View Tab > Uncheck 'Use Simple File Sharing'. Make sure you can ping the router, dns server and the 2003 machine. You may want to setup shares, do this through the console in admin tools. Be sure on the network connction of the XP machine you also have file 'File and Printer' sharing enabled. Be sure the Server & Workstation services are also set to manual.