client Systems gets hanged up during a particular time period

Hi,
I have a problem..in my network i have a windows 2003 server(its machine config is same as the client machine) with around 12 machines connected to it.All the client machines run winxp sp2.When the clients try to login to the access the shared folders of the server,Their system gets hanged up.This happens usually when all of them try to login within a time period of one hour.Then i have to disconnect the sessions manually on the server.Is there anyway to stop this practice and allow them to login normally and without getting stuck.Also can u suggest a method for making the network response faster.We use a 100Mbps Lan card but it does not use its full potential.Pls help me