Disconnect LAN while connect to internet

Hi

We are using Windows 2003 sbs server for our network. we installed broadband connection in one of our client machine. the problem is while we are connecting to internet this client machine losses its connection from server say client machine is totally isolated from network.after disconnecting from internet and restarting the client machine then it will restore the network.
can anyone help us to resolve this issue??

thanks in advance

syban