I have same h/w on slack platform. This is what worked for me.
1) On the XP box: Printer> Properties> Port - Then uncheck 'Enable bi-directional support'
2) in CUPS: Device URI: smb://Workgroup:user@password/xpboxname/printer-sharename
(where 'xpboxname' is the "Full computer name" on the -Computer Name- tab of your
System Properties, and 'printer-sharename' is the "Share name" on the -Sharing tab- on
your printer properties. "Workgroup" can also be found on the -Computer Name- tab)
-Unnecessary statement: System Properties can be found by clicking Start> right click
my computer> Properties.
3) in /etc/samba/smb.conf -uncoment line: "Printing = cups"
You got everything else configured. That should work. Let us know.