Windows Messenger 5.1 new sign-on problem

Can we keep using Windows Messenger 4.7, 5.0 and 5.1 in on o ur Windows XP
desktops at work for the foreseeable future? The interface is SOOO much
cleaner than the Windows Live Messenger.
In the past week or so we've have lots of problems with people not being
able to sign in:
1) The automatic signing-in process just keeps going without ever signing
the user in. They have to cancel sign-in and sometimes fully exit the program
before manually signing in.

2) Users are repeated being prompted to enter a display name with the
message: "Please provide a display name so you can sign in. Do not use your
e-mail address or leave the box blank. We now require a display name that is
not your e-mail address to protect you from e-mail virus attacks."

They enter their user name and are prompted again the next time they log
into their computer.

Any help on how to solve these would be greatly appreciated! Thank you!