I had this exact same problem. Thanks to David at Roku, we were able to trace the problem to the messenger service being disabled on my XP box. It was a clean install so I'm not sure how it got disabled. All service packs were applied.
Hope it helps.
The messenger service is disabled by default, starting with XP SP2, since most people don't need it, and it has some known security problems.
However, I'm not sure what the messenger service has to do with XP shares being visible to Samba clients. It *should* have nothing to do with this. Must be some side-effect of starting Messenger Service that broadcasts your machine/share info? Hmm...