PC lost Workgroup accessibility

Hi all

One of my clients has a simple workgroup consisting of three Win XP computers.
The network worked fine for a long time, but one pc could suddenly not be accessed by the rest of the network. The faulty pc however can still access all the other computers. Nothing on this box changed. I cannot ping the pc or access any of its shared files.
I have made the user a ?Power User, Administrator etc?, but have no luck accessing it.
I have disabled the windows Firewall and disabled Norton.

Any ideas would be appreciated?.
Regards
Jacques