Can't display all computers within my peer to peer workgroup.

I have various issues communicating among three computers (win7, XP, win98) in a workgroup on my peer to peer home network. The network has functioned well in the past but apparently I have changed something.

One symptom -- right now I am looking at the workgroup with Windows Explorer on the XP computer. With Windows Explorer I select the workgroup icon and click its (+) in the left pane. All three computers display below the workgroup. But only two of the three computers display in the right pane.

Explorer on the XP.
LEFT PANE:
Network Places
__Entire Network
____MIcroSoft Windows Network
______Servicco
________Homebuilt PC (Homebuilt) OS=XP-Pro, Ethernet to router
________Dell Studio 15 (Studio15) OS=WIN7-Pro, laptop, WIFI to route
________Northgate (tower) OS=WIN98, Ethernet to router

RIGHT PANE:
________Homebuilt PC (Homebuilt)
________Dell Studio 15 (Studio15)

Windows Explorer on the WIN7
(It sees only itself (Studio15) in the left and right panes of Explorer)

Windows Explorer on the WIN98
(It sees only itself (tower) in the left and right panes of Explorer)

What could be my problem that the three computers cannot see each other?

After about a half hour after posting the previous post I checked the screens of the XP and the Win7. Both Windows Explorers have changed. On the Win7, the XP icon now displays in both windows and its folders and files are accessible.

On the XP, the Win7 displays as before and its folders and files are accessible. But the Win98 no longer displays in the left pane as before.

On the Win98, it still displays only itself.

Note: Earlier, on XP the Win98 icon displayed in the left pane only. But by high-lignting the Win98 icon in the left pane I could display in the right pane the Win98's folders and files (those that were shared) -- as they should.

After being shut down overnight:
All three computers now recognize each other and files are mutually accessible. I didn't change anything.

After I started this thread:
I read the sticky post "Basic Requirements Before Posting your Networking Thread". Here some of the additional information it suggests:

Internet provider = Comcast (cable for computers & TVs & 2 phones.)
Virus scanner = FProt.
Firewalls: Microsoft.

Here is the physical setup:
In my basement:
Modem Comcast/rented Motorola SBV5220 digital voice modem. (in=coax, out=2 phone lines. This may not relate to the computer network.)

On 2nd floor near the two Ethernet-connected computers:
Modem Motorolla SB5120 Surfboard Cable Modem
Router Netgear WNT2000

Here are the Attached Devices according to the Netgear WNR2000 router:
# IP Address MAC Address Device Name
1 192.168.1.9 00:21:6A:4F:F7:9E STUDIO15
2 192.168.1.8 00:04:5A:80:A4:2F TOWER
3 192.168.1.2 B0:79:94:4C:98:72 <unknown>
4 192.168.1.4 00:50:8D:82:877 HOMEBUILT
I don't know what #3 might be?

On Windows 7 Explorer the Network Infrastructure contains 3 items as follows:
1 Internet Gateway Device
2 PK5000Z AP
3 WNR2000
I don't know what #1 & 2 might be?

Is there anything in this information that could be the cause of the intermittent inability of my computers to connect to each other?