How do I get my working network back?

Up until about two weeks ago I know that I had a working network of three
computers running WinXP and two running 32-bit Vista. I could get on any of
the XP computers and access files in shared folders on the Vista machines.
I could go to any of the computers and click on "Network" and see all five
computers. I didn't use the network every day but it was there when I
wanted it. Yesterday, I was on one of the XP machines and wanted to open an
Excel file that was in a folder on a Vista machine and I couldn't access the
network. I looked on the Vista machine and the file was there but I
couldn't see any other computers on the network. Nothing was working like
it had been the last time I looked. I hadn't changed a thing--no new
software or hardware! Nothing!

After changing some firewall settings and adding some protocols on the XP
machines I can get them to see each other and access files among them. I
can get the two Vista machines to see each other and swap files between
them. However, I can't get any virtual connection between the Vista
machines and the XP machines. They're in the same workgroup but it's like
they don't know the other part of the group exists.

All five computers can access the web through the router and DSL modem so I
don't think it's a hardware problem.

BTW, all the machines have the latest Windows updates--SP#3 on the XP's and
SP#2 on the Vistas!

Can anyone help? TIA!

Gary