Can't Access One of My Network Computers via Windows (File) Explorer

Question

I've just installed a new router (Linksys EA3500 Dual Band) to replace my old (Linksys WRT100), but can't imagine how that could cause this trouble, except that prior to installing the new router, all worked properly.  I have three Workgroup computers
on my network, one running Win 7 Ultimate 64bit (call it Tower-755, an older laptop upstairs running Win 7 Pro 32bit (call it MommaDell), and a super laptop running Win 7 Pro 64bit (Call it M6600).  On Tower-755 I have a mirrored Raid1 drive - call
it Z: - that I share over the net (i.e. Everyone = Read/Write).
Since installing the new router, when I open File Explorer on M6600 or MommaDell, I can see - in the search pane (under Network) - all three computers, and when I click on MommaDell, its shared folder is accessible, but when I click on Tower-755
I get an error saying, "Windows cannot access
\\Tower-755, yet I can ping it successfully from a command prompt.
However, when I open File Explorer on Tower-755, although I can see all three computers in the search pane (under Network), I can't access any of the three (including Tower-755 itself); I get the same error message, although I CAN ping them from a command
prompt.
Also, and I'm not sure if this related, in the search pane under HomeGroup on all three computers, I can only see MommaDell.
I've checked and double checked all my router settings, including the IP Address range (192.168.1.2 - 51).  I also have a Domain Controller server with two joined workstations on the net, so I have a static IP in order to be able to access the server
and/or workstations remotely when we travel.
That's my story and I'm stuck with it, but I'd sure be grateful for any help with this mess.
Capt. Dinosaur

Answer 1

Hi Capt,
In order to help you analyze your issue, we need confirm some situations.
1. Did these 3 computers all join the same HomeGroup or Workgroup?
Yes
2. Domain Controller server with two joined workstations you mentioned, did it mean two of these 3 computer joined the domain environment?
No.  Those are separate computers in their own domain, just using the same network.
3. Did you ping via IP address or computer name?
Both, but this morning something is different. 
  a.  MommaDell and M6600 can ping each other both by IP and Name, but can't ping Tower-755 either way.
  b.  Tower CAN ping both MommaDell and M6600 both ways.
According to your description,
Tower cannot ping MommaDell and M6600.
No, Tower CAN ping both MommaDell AND M6600 both by ip and name.
MommaDell can ping Tower.
M6600 can ping Tower.
Both of the above was true yesterday, but not this morning.
Please check if MommaDell and M6600 could ping each other.
Yes they can
And make sure all of them turn on the network discovery and turn off the firewall temporarily for test:
Network Discovery is ON in all three machines, and Windows firewall is always off because I have ZoneAlarm on all three machines.
Enable or disable network discovery
Which?
http://windows.microsoft.com/en-in/windows/enable-disable-network-discovery#1TC=windows-7
If it doesn't resolve, please post every computer's ipconfig /all output here for analysis.
it's 7:30 and I have to be out until 2:30.  I'll do the posting then, and thanks for the help.
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Capt. Dinosaur