Can you ping the Windows XP box from the Ubuntu box and vice versa, that is, have you verified that they can see each other? Are they on the same subnet (I'm certain they are but it's something that should be verified also)?
It's been a while since I had XP. Have you installed
netBEIU to the XP box? As I recall, I had to do that. I think I also had to set up a login user and a password for the XP box and enter them in my /etc/samba/smbpasswd file, but, as I said, it's been a long time.
A web search for "network windows xp linux" will turn up a number of links. You might also take a look at
Samba by Example (PDF). That's the guide I followed.