Computers missing in Network or My Network Places

I have about 20 machines on my network. Yesterday I noticed on my Vista Business machine that when I open the Network window the only machines that show up on it is my computer along with my partners Vista machine. All other computers and servers are no longer listed. I can ping them. All of my mapped drives are working fine. They are just not listed in the Network window. I thought it might just be my machine but I am finding out that it is network wide. What do I do to correct this???