Unable to access shared folders in a workgroup across a network

Hi experts,
I have a small workgroup, consisting of six computers running on windows XP Pro edition.
The network has been ok for as long as I can remember.
Then today when I try to access some information on shared folders from the network, using any of the machines, I get the following message,

Microsoft Windows Network
Workgroup is not accessible.
The list of server for this workgroup is not currently available
I have a printer connected to one of the machine, and this is accessible from any of the other machines.
Which means I can print from any PC.
Windows firewall is off, in each of the machine. It seems TCP/IP settings are ok as all the PC's can surf the internet and ping each other. I have run the “network diagnosis wizard” and the “Configure work network settings” wizards on all 6 PC’s several times, but nothing seems to work.
What do I do, in order to access files from shared folders across the network?
Thanks for your answers.
