Dropping Connection With a Windows 2003 Server

I have an iMac running Maverick connected to a windows server 2003 through a modem router. The connection keeps dropping every few hours and when I want to reconnect I get the message "Check the server name or IP address, and then try again". I have to reboot to get my connection back. I also have a windows based computer connected to the server in exactly the same way and have no trouble with that.

Can anybody help please?