Solved, but I don't know what exactly 'solved' the problem.
For future searchers - I feel your pain. This kind of issue is posted a hundred times a day in various forums.
Microsoft just didn't do their homework IMO.
Why, for cryin' out loud, would they hide the Workgroup settings, and not even show a live Workgroup under the Networking & Sharing control panel app? ... I would love to hear the rationale.
For whatever reason, I was able to get a Logon from one of the Windows XP machines to the notebook loaded with Windows 7 Home Prem x64. I logged in. Once that succeeded, then both Windows 7 machines suddenly were able to detect/identify all the windows xp systems. From the Windows 7 Home Prem system I was then able to access shared files on each of the windows xp systems [why?].
However, the Windows 7 Ultimate x64 desktop was still returning errors if any Win XP system attempted to logon. The Guest account on that Windows 7 system was enabled. [for the first time ever - I just enabled that one yesterday while trying to solve this]. With the Guest account enabled, the error message changed to "User has not been given permission to access this resource". So today I turned it off. Then, thinking the issue might be IP stack or driver
related, I installed a USB wireless adapter in addition to the hardwire that the desktop Windows 7 system was using. I still was unable to access the system. So I disabled the wired adapter and rebooted. Still unable to access, but the error changed to "network path not found".
I rebooted again, re-enabled the onboard hdwe NIC, and fetched a few Windows 7 updates I wanted. After that started I went back to work on the network issues. From one of the Win XP clients [this one happened to be Home Edition] I accessed the belligerent Desktop and suddenly got a good Logon popup. Logged on with the main account credentials, and it worked.
I have no idea what the underlying issue is, but it worked. It must have something to do with Windows 7 networking policies that differ for wired vs wireless access.