With standard, Old Style Sharing, you should not be prompted to Join anything. You should be asked to type in a Username and password for that username to gain access to Everything that is Shared on the other LAN computer.
I own multiple computers, several in my home, desktops and a couple of notebooks both Windows and Mac, running a mix of XP and 7 along with OS X 10.7 and 10.8 on the Mac. I can get to every computer and every file on all those computer.
I have the same username and password on all of them, as they are all mine so why would I want a different username and password on each, but even when I take a system home to work on (part time Tech Support for friends/family and paid) all I have to do to log on to any of my personal computers is to enter my username and the password and I'm IN. Everything I have shared is accessible to me from that other system. Same goes if I need to go from one of my PC/Mac to the system I am working on, IE enter the Username and password for that computer and I have access to everything that is shared on that computer.
I personally have never worked with the Windows Homegroup system or used the XP version, Simple File Sharing. To me neither of those things are simple as they Restrict access to what MS THINKS should be shared and nothing else.
Also if you are running some type of Network/Internet security/virus software they are notorious for restricting access and doing it Willy Nilly at any time. Even turning them off sometimes does not stop them from blocking access to LAN computers.
I don't know maybe I have been lucky but I have never had any problems sharing, accessing, computers using the tried and true, old school, system. That is why Ethernet was first invented, to Link computer together over a Local Area Network.