Unable to Browse Network

Sometimes when I log into my network, I am not able to see the other computers when I go into Network Neighborhood. I am able to ping the other machines. Only after rebooting am I able to see the other computers.
Any ideas on what the problem couldbe?
Just to rule out the obvious, there are shares on the other machines (server&workstations) and Workgroup/Client for Microsoft are set correctly, and I am able to log on to the NT server.
It happens intermittantly so maybe windows is just weird.

Thanks in advance for your help.