Check the firewall on the inaccessible system. Probably more than 90% of the time, these errors are due to a misconfigured firewall.
My network consists of 2 xp pros (desktops) and an macbookpro (wireless laptop). The problem I'm having is that one of the desktops cannot be accessed on the network by any of the other pc/labtop. To be more specific the mac is telling me that the alias is incorrect and on the pc I'm getting an error message saying that I don't have the permissions. I am an administrator on this network and believe I have tried everything possible like checking that the pcs are in the same workgroup and the permissions are correct. I've gone to apple's website and tried the suggestions there for the mac but still get the error. I'm starting to thing that there is a setting on the pc that i'm trying to access that is preventing me from accessing it, but i can't find it. Any help would be greatly appreciated!