Virus Problem

My question is ! I m working in an organization. where i m facing virus attacks of w32.Nimda and Klez . I also have downloaded its removal tools from symantec web sites. But the problem is still prevailing. This virus is coming from emails . when a client open his mail . virus alert appear and the users on clients are not much aware about computer operations. so they run all the files dont quaratine. So problem is going on . Plz give me useful suggestions if you have in this regard. Thnaks.