i have had the same problem with both windows messenger
and msn messenger. after going through my router's
configuration i tracked it down to being Port 1723 being
forwarded to my VPN server. once i turned the port
forwarding off for that facility it just started working
again as if nothing had ever happened. to prove this was
the problem i signed in and out with the port forwarding
on port 1723 off numerous times. then turned the port
forwarding back on, and sure enough msn messenger and
windows messenger would both instantly hang on sign in.
to this day i have not found a way around this issue, not
even with the use of Microsoft ISA Server 2000 and
connecting to msn using the HTTP proxy. so to cut a long
story short, if you are running any servers from your
connection try turning the port forwarding off temporarily
and see if that fixes it up.
Andrew Parsons