Vista file sharing across and Access Point / Networks / Merging, etc.

I have a simple LAN that includes a wireless access point device.
Some computers are wired and some computers are connected via the AP /
All computers are on the same subnet as you might well expect.
Most computers are XP Pro.

We just introduced a new Vista system on the wireless part of the LAN.
The Network and Sharing Center settings were normal for our Vista systems:
"private network,
Network Discovery ON,
File Sharing ON,
Public Folder Sharing ON,
Printer Sharing ON,
Password Protected Sharing OFF,
Media Sharing OFF.

We shared the C: drive and the following observations were made:
All the computers on the LAN could see and get into the Public Folder
Only the wireless connected computers could see and get into the shared C:
The wired computers could see but could NOT get into the shared C: drive ...

I notice in other LANs that the wired computers in Network and Sharing
Center are connected to "Network"
I notice that this wireless Vista computer in Network and Sharing Center is
connected to "[ssid]" as a network name - so I suspect that a wired Vista
system on this same LAN would be connected to "Network".

I found the control that will "merge" networks and merged the SSID-named
wireless network with the "Network". That didn't help.
I added Everyone to the permissions on the Vista shared drive. That didn't

Eventually, I added Guest to the shared drive permissions and it fixed the
problem. Now all the computers on the LAN can see the shared C: drive and
can get into it.
On other LANs I've not had to add Guest in order to gain access to shared
folders such as c:\Users\Me

I wonder why having the wireless segment matters here?
I wonder if the C: drive is handled differently than a subordinate folder in
such a case?

I need explanations that will allow me to reliably network Vista machines!