Slow network

Pliz help. I administer 12 computers that are networked and all connect to the internet via an adsl modem. I have a win2000 server where users save thier files to. The problem is that it takes a very long time to open simple word and excel files that reside on the server. It also takes a along time to save to the server, even if the file is very small. I have blocked internet access for some users by entering 127.0.0.1 in the proxy settings of internet explorer so that they can only access e-mail. the slowness happens at different times for different users. Pliz HELP!!!