reinstalling , system restore or anything else is not gonna help so don't wack your brains
for ones having problems the best solution is to use another client for the time being.
i think it is the server that your email i.e. your passport is registered on to use msn messenger (i have the same problem)
a few servers are having troubles
but u can connect to ur messenger using clients like trillian , integrity messenger etc and it will work just fine ....
i don't know why but i also have the feeling it has something to do only with windows xp .... (i maybe wrong here)
the msn status website has said tht the service is experiencing difficulties ...
which says
Sign In
The .NET Messenger Service is temporarily experiencing difficulty. You may be unable to sign in. Please try again later.
Last Update: 7/3/2004 6:06:00 PM Pacific Time (GMT -8:00)
so lets hope its resolved asap and we can start using msn messenger and Msgplus