Problems with excel when plugged into the network

Can anyone please help us ? When our computers are plugged into the network it takes around four or five minutes to open excel documents. If you unplug the network cabell before atempting to open the files they open imediatly. The computers that the problems are occuring on are Toshiba satelite pro A120-159. There other problems when plugged into the network such as being generally slow and unreliable……

Please help !!!!!