Denial of Service?

Recently we are having users logon to our NT 4.0 network, to get a message of “password not valid”, regardless of who logs on. Deleleting the pwl file from Windows does not result in a request for a new password. Servers and print service are denied, but internet access and mail are still available. We find no common links, yet. Our virus files are up to date. Only one of the affected workstations uses Outlook, and none use IE5. W e use Netscape 4.6 and above for our e-mail client. We have reloaded windows, tried deleting network services and reinstalling, deleted the user from the server and re-opeded the account, all with the same result. Thursday there were 3 stations with this problem, now 9 (monday). Any ideas?