I am not using a username and password. I have two users on my XP box and neither of them are password protected. In the roku setup I have not username and passwords to accesss the shares, that is why the "user/pass ok" part of the messege is puzzling to me.
Sounds like the roku is accessing the xp box using the guest account. The guest account isn't attached to any group in the XP version of windows (use to be part of the users group) you could try granting read permission to the guest account on the shares (may also have to grant it on the folder in windows too(security tab in folder properties)
All just a guess mind you.