Ok, so I ran into this networking problem last week, and it took me a bit to figure it out, so I'll explain the problem and how I fixed it here.
You are logged in as a user on machine A. You try to view shared folders on machine B. You get the logon failure. That's because the user is not allowed to log into or share files with machine B. Add the user to the remote users group on machine B. The machines have to be in the same workgroup or domain for you to do this.
I recently got a brand new Windows 7 desktop to work from home, but I also have this old Windows XP desktop at home that I've been using for personal stuff and remoting into work. I wanted to give my work computer access to my home computer so that I could just copy some of the files (The Longest Journey! :D) over as needed. My new win 7 desktop is MACHINE_A, and my home desktop is MACHINE_B.
The problem I hit was that when I tried to browse to \\MACHINE_B, MACHINE_A would say the logon failed because the user had not been granted the requested logon type at the computer. The thing that really confused me was that when I first tried to explore \\MACHINE_B, it would prompt me for a u/p, and I would put in the one for MACHINE_B\admin, and it would churn for a moment, and then it would give that error. Here's what happened.
MACHINE_B was on workgroup settings, and MACHINE_A was on the work domain. I was logged into MACHINE_A as DOMAIN\userOnA, a user that was unknown to MACHINE_B. MACHINE_B gave remote access only to administrators, which is why userOnA was blocked, even with the correct u/p. I had the right authentication, but not the right authorization. I tried to add userOnA somehow, but I couldn't. I probably could have opened MACHINE_B wide open so that anything could access it, but I didn't want to do that.
Eventually, I figured out that I needed to put MACHINE_B on the work domain so that I could select user objects from it, and add them to the remote users. That would let me add userOnA to MACHINE_B's remote users, which means that if userOnA were authenticated, then he could also perform remote actions like sharing files from MACHINE_B. I joined MACHINE_B to the work domain, logged in as admin, added userOnA to the remote users, went over to MACHINE_A, explored \\MACHINE_B, and I was able to see the shared folders.
Actually, to tell you the truth, I could be mistaken about how I explained some of these things, but based on what I did and what worked, these are the concepts that I drew. In any case, I hope this helps you out!