Connection Problems – Server 2003/XP Pro

I’ve got a server running SBS 2003 and 9 clients running XP Pro. The client computers are left on 24 hours a day, 5 days a week. The problem is that it seems every night the computers lose their connection to the server. In the mornings when the users come back to the client computers, they have to restart in order to reconnect to the internet/server based programs/network shares. What could be causing this?
Thanks for your help.