Win 2000 client hangs when connecting to server

I am running a Win 2000 Advanced Server network with approximately 20 clients, all of which are either XP Pro or Win 2000 Advanced server. I have the following problem. As the day progresses the clients have a more and more difficult time connecting with the shared drives on the main file server, until it begins to hang up the client entirely. Rebooting the client clears the problem, which then emerges as the day continues. Any idea what this could be?