Ok, time for someone else to solve my dilemma.

Short:
Accessing one of my Windows PCs over the network from my Mac or network-connected settop box requires authentication (user/password) and for the other system (identically set up as far as I can tell), it doesn't.

Full:
I have two Windows machines sitting right next to each other and connected to the same local network. Each is set to the same workgroup and as far as I can tell they also share pretty much the same LAN configuration. Guest account is disabled on each machine and every shared drive/directory is set with Sharing permissions with both the "Everyone" and default user (with admin access) to have "Full Access." I've even got the Security settings on both computers matching up, each containing the "Everyone" group with full access.

Both are running Windows XP Professional SP2. I'm not certain at what state of additional updates each one is. They both currently have auto-update turned off.

I can see and access both of these machines from each other using UNC paths. Typing \\machinename works from each machine to the other (as well as to themselves). This will list all shares for the connected computer.

I can access both machines from my Mac as well by using the name/password of the primary user I mentioned above, again showing me all the shared volumes.

Here's where the issues start...

I can access one machine from my Mac as a GUEST - that is, without having to authenticate. I can't do this for the other. It gives me an error saying that machine doesn't accept guest connections.

Likewise, from my new SageTV Extender settop box (also on my network and with the same workgroup setting) I can access one machine without authentication while the other needs name/password.

I'm totally stuck and can't figure out rhyme nor reason why access to the two machines differs in this way while they appear to be set up identically.

Both have the built-in firewall active and set the same way. Both are set up to log in automatically. Both are set to allow remote connections (and I successfully use Remote Desktop Connection with both).

As I was writing this message I noticed that in fact I was missing the "Everyone" in the Security settings for each of the shares on the problem computer. I've since added those with matching settings (full) to the other computer. No dice, same issue.

Help?