Network access problem

I'm running a peer to peer network, with
fifteen computers. I have Vista on my computer,
all others are XP prof.

I can access other computers on the network
through shortcuts or from my network places on
the XP computers, but I can't see anything but
myself when I try to view the workgroup.

Some computers can see everything in the
workgroup while others only see one other
computers.

The network has been working fine for quite
awhile.

Any help would be appreciated.