Sorry to report that the registry modification does not seem to work.

When you're signed into Windows Messenger, you can see the IP address listed
in HKCU\Software\Microsoft\MessengerService, so I edited it to match what
WindowsUser2009 suggested: messenger.hotmail.com;64.4.34.200;1863 (mine
originally had a different IP address - everything else was the same.

I wondered about this, because when you're NOT signed into Messenger,
there's no IP address after messenger.hotmail.com in the registry. So,
anyway, I made minde 64.4.34.200 and then signed out and exited .... so far,
so good. The IP stayed in my registry and I was able to start Messenger again
with no problems.

Next, I rebooted my computer to test further. To my disappointment, I was
right back to Messenger trying to start, the icon flashing with no success. I
canceled sign-in, exited the program, launched it again from Start, Programs,
Windows Messenger, and was now able to sign in.

Another little trick that worked was the following, which makes me wonder
whether this issue is somehow tied into Kevbirder's suggestion .... not sure
how, though. Here's what happened:

After I canceled the automatic sign-in that was NOT successful, I chose
"Sign In As..." and re-entered my password. It worked! WHY??

Take-away from this to me is that automatic sign-in does not work for
Windows Messenger version 5.1 (others?) any longer. Any ideas?