I'm an thernet engineer so this should be easy..............
If you can PING from both ways (ST to PC and PC to ST) then 99% of your work is done... I think the easiest and safest way to do this will be load a FTP server on to XP (make sure you turn the firewall OFF until you have this working), you can download any good FTP server (GUILDTFTP i think is still working) then load an FTP client on to the ST with ST software, now i am unsure if this exsists so thets your next step.
Once you have both set up, then you can use your ST to access the XP FTP server using the ST software to connect to the XP box via IP (ftp;//xxx.xxx.xxx.xxx:port).
Many Thanks
Ashley