Some progress! Apologies for long post but a few things are going right now...
After adding 192.168.1.109 and 192.168.1.113 as trusted sites in XP's PC-Cillin, and typing in KevinJ's suggestion of:
iptables --flush && iptables --delete-chain (note, I needed to re-boot after this to get my internet back)
I can now from linux:
Ping myself (Yes)
Ping XP (Yes!)
and from XP I can ping myself, but I cannot ping linux from XP.
In addition (and this is exciting!) I can SEE linux inside XP's network neighborhood. If I try to browse it, I get an error:
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\\Localhost is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissons.
You were not connected because a duplicate name exists on the network. Go into System in the control panel to change the name and try again.
===============
I have tried changing XP's name as per the message btw.
But wait, there's more!
In konqueror if I navigate to
smb://192.168.1.109 (XP's IP)
I can SEE XP's shared folder! However, if I open it, it appears empty, although it is not.
Incidentally, if I navigate to smb://192.168.1.113/ (myself) I get an empty screen. No error, but no files/folders.
I wonder what to try next?