Hmmm.... that is odd. Sounds like you're doing everything by the book. Let's start from square one - I know you probably did everything correctly the first time around, but use
this guide as a checklist, and try resetting the shares from scratch.
We know it's not a network problem because the shared machine can't access the files either (you were right to check your NetBIOS and firewall settings otherwise). I don't think I've ever seen the establishing user account not be able to access the shares - I'm thinking something must have gone wrong in the process. Really out of the ordinary. Let me know how the reset goes, and we can go from there.