|Yes, I did all that. I've actually now got round the problem.|
On the 2003 server I enabled Guest logins and this did the trick. I got access to both the shared folders and the printer from the XP machine.
Noting that allowing Guest access was actively discouraged and concerned about jeapordising security, I disabled it again. However, I've still got access from my other machine. Hurray!
Thanks for your input, though.