Random logon screen

Hello. My work is still running a broadband section on our
Network (unfortunatley). We are using old 16-bit UB NIU\pc
NICs and UB modems. The problem is that on startup,
sometimes we get the Networking logon screen (user name,
psswrd, domain) and sometimes we get just the regular
Windows logon screen. If we logon through just the Windows
logon (or just cancel it) the users can still access e-
mail through our Exchange server, but all other domain
servers and services don't work-which strikes me as rather
odd (unless 98 has Exchange reconnect procedures running
in background). When we're lucky enough to get a Network
screen, all network resources run fine after login. Our
department is stumped. Any thoughts from anyone?