Windows XP home on all 4 home computers. Networking been working for years.
Installed firewall on just one computer, my main workhorse. Network is through router, through Time Warner cable modem.

All computers can ping each other, including the one with firewall.
The one with firewall does not show up in WORKGROUP on any computer, including self.

Other computers can net use to shares and get them. Yes it works, however the fire walled computer does not show up in WORKGROUP.

Here is what I think is important.
When firewalled computer tries to see WORKGROUP computers, it is denied access to view the WORKGROUP. "Workgroup is not accessible. You might not have permission to use this network resource....etc. Contact administrator ...access permissions."

I AM the administrator. I don't have a clue! Somehow installing the firewall has removed my permission to access my own WORKGROUP., yet is is working via net use commands.

Net user guest /active:yes has been performed.

More information. I hope it's useful.
The machine with the firewall is: King-Daddy.
The machine I am testing from is: Princess2

The is "Browser not active" on King-Daddy. The Computer Browser service is started. I don't know where to go from here.

King-Daddy:

C:\WINDOWS>browstat status


Status for domain WORKGROUP on transport \Device\NetBT_Tcpip_{A051A54C-8209-4FAB-B279-ACA549622387}
Browsing is NOT active on domain.
Master browser name is held by: KING-DADDY
Master browser is running build 2600

PRINCESS2:

Status for domain WORKGROUP on transport \Device\NetBT_Tcpip_{7CB8ED3A-8987-4CCB-B619-EE5345504F4B}
Browsing is active on domain.
Master browser name is: KING-DADDY
Could not connect to registry, error = 53 Unable to determine build of browser master: 53
\\\\KING-DADDY . Version:05.01 Flags: 51203 NT POTENTIAL MASTER
1 backup servers retrieved from master KING-DADDY
\\KING-DADDY
There are 6 servers in domain WORKGROUP on transport \Device\NetBT_Tcpip_{7CB8ED3A-8987-4CCB-B619-EE5345504F4B}
There are 0 domains in domain WORKGROUP on transport \Device\NetBT_Tcpip_{7CB8ED3A-8987-4CCB-B619-EE5345504F4B}

Why? Browsing is NOT active on domain.
Master browser name is held by: KING-DADDY

What I did that resolved the problem...

IPCONFIG /ALL did not show any DNS settings.
I am configured TCPIP "Obtain DNS server address automatically" but that was not happening. I don't know how or what firewall installation affected, but I used to be able to see the DNS that was fetched form the Router. Not now, but web browsing still worked.

In TCPIP properties/Advanced I manually added my two DNS addresses.

Now I get:
C:\WINDOWS>ipconfig /all

Windows IP Configuration

Host Name.. . : KING-DADDY
Primary Dns Suffix.. . :
Node Type.. . : Broadcast
IP Routing Enabled.. . : No
WINS Proxy Enabled.. . : No
DNS Suffix Search List.. . : 74.118.212.1
207.69.188.185

Ethernet adapter Local Area Connection 5:

Connection-specific DNS Suffix . : clackey.nc
Description.. . : Realtek PCIe FE Family Controller
Physical Address.. . : 40-61-86-85-F8-45
Dhcp Enabled.. . : Yes
Autoconfiguration Enabled.. . : Yes
IP Address.. . : 192.168.1.102
Subnet Mask.. . : 255.255.255.0
Default Gateway.. . : 192.168.1.1
DHCP Server.. . : 192.168.1.1
DNS Servers.. . : 192.168.1.1
Lease Obtained.. . : Monday, January 17, 2011 9:42:51 PM
Lease Expires.. . : Wednesday, January 19, 2011 5:02:51 PM

The first DNS is one of the Clear Cloud Malware filters.
2Nd DNS is one of the Earthlink DNS.