Stopping SPAM

Can anyone help me? Our Exchange 2000 server is being used to relay SPAM in massive amounts and we can’t seem to get it stopped. It is a Win2k server. We are in process of getting all our security patches up to date, but we have almost all of them done already and are still being used. We also went to MS website and printed their instructions on dealing with the problem, but it didn’t take care of it. ?? Help?