Assuming you tried the Security tab thing sow2 suggested, it sounds like you've got everything setup correctly; it may be a problem with the guest account on the Windows box. I once had that happen on my XP box, and fixed it using this
, but I'm unsure how well it'll go under 7 (nor if the same thing has happened to you).
To be clear on this, when trying to log on as a guest, don't enter a username/password at all.
Also try googling for the "xbmc4xbox" site, assuming you haven't found that site already, there some more stuff that might be of use to you in the guides section of the forums.