Network of a group get dumb&unaccessible

In our network of win9x & NT server platform we have faced a typical problem of a single group of users of NT gets hanged & unaccessible from the local network. On checking each machine of the group, one machine is found dead or hanged and on restarting this machine the whole group gets its network live and accessible.Please suggest the cause of failure if any one has faced it. And provide solution to prevent it if anybody knows it.