Windows 7: Mixed Win7 & WinXP network no longer sharing, tried lots, please help

I am having issues with my home network. The following is as complete a description of my
network and attempts to resolve my problems as I can muster at this time. If more info is
useful, please help me to identify what is germane. I am sorry for the longt and detailed
post, but I don't know what else to try.
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The basic configuration is:

Internet<=>ISP Mode/Router<=>Gigabit Switch<=>Win 7 Desktop &Win XP Desktop

Wireless Connections<=>2 Win 7 Laptops

Wireless Network Printer

or:

Internet connection via ISP provided 2Wire modem/router. Internet is functional on all
computers.

All computers can print to the Network printer.

Linksys Gigabit switch connects the two "problem" PCs.

Windows 7 Desktop can "see" both Windows 7 Laptops in "Windows Explorer>Network". (There are no
shares amongst the Windows 7 PCs.)

Windows 7 PC (mainly used for email games and surfing, also used as data server) cannot see WinXP
PC shares.

WinXP PC (Used for music production) cannot see Windows 7 PC shares.
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First, my home network was working fine until a few days ago. With the holidays, I am not
exactly sure which day, but certainly less than a week ago I was able to share files and
stream media between the Windows 7 and WinXP desktops.

Here are the things I have tried:

There is no Homegroup on the Windows 7 computer. (I did have a Homegroup setup for shares
between the Windows 7 Destop and my wife's Windows 7 Laptop, but I disabled it to try and fix the
issues. We don't really share a lot of files, and I am using ThumbDriveNet to share the
occaisional file she needs. If I can fix the issues between the Windows 7 and WinXP PCs I may
try and enable the Homegroup again in the future.)

Both computers are using a workgroup named "WORKGROUP".

Each computer has a unique name.
I use Indian Deities for my computer names. Windows 7 = SHIVA
WinXP=SARASVATI

Network Discovery is turned On in Windows 7 (not sure how to do this in WinXP).

File and Printer Sharing are turned on in both Windows 7 and WinXP PCs.

I have tried Pasword Protected Sharing turned on and off. I created User Accounts that are
identical (Name & Password) on each PC.

I disabled TCP/IPv6 (and since re-enabled it).

I have rebooted the gigabit switch.

On the Windows 7 PC I have switched between Home and Work networks and back.

I can ping each computer successfully from the other computer (i.e. Windows 7 can ping WinXP and
WinXP can ping Windows 7; no packets are dropped).

I have tried to access each PC using the IP address in Windows Explorer to no avail.

I have been working on and off on this issue for several days (less than a week, but getting
close). I have scoured various forums and attempted to use each suggestion that I have
seen.

HELP! (Sorry for yelling, but the frustration factor is quite high.)

I have been using these computers together for a couple of years. When I first got Windows 7, I
had some minor difficulties getting the network up and running. But it has worked perfectly
until a few days ago.

Windows Update History shows:

Security Update for Windows 7 x64-based Systems (KB277953842) Successfully installed on 12
-21-2012 (uh-oh, could the Mayan Calendar have been talking about my network?).

I also have a few Windows Defender Definition updates from 12-14-12 on, but I don't think
they would be part of the issue.

Thanks for "listening" and thanks for any assistance.