SBS Server 2003 slow login and slow connection

I have i little problem in the company with our server connection. It takes time to login to our domain SBS server 2003 and opening a documents files. Two weeks ago, I made a changes in our server. I disabled the DHCP I reconfigured it. The server has two NIC card, but i only use one NIC for our internal LAN. The set up is from modem of our ISP going to switch and distributed to all workstations for internet connection. The server is configured as DNS server and our Exchange Server. Everything is doing fine except of extremly slow during login and accessing the shared files from the server. Hope I stated it clear and hoping for your help.