Error in signing on to messenger (long)

I have a relative who pretends to be mentally ill and very stupid so I have
to use remote assistance to do maintenance on his computer, running XP Home
SP2.
He had a problem whereby he couldn't sign on to Messenger in order to
initiate a Remote Assistance call. We then found that he couldn't log on to
Hotmail either so we did all the stuff of clearing cache, cookies, temp
internet files, adding hotmail.co.uk as a trusted site etc etc, and he can
now log on to Hotmail.
However, if he tries to sign on to Messenger using the exact same password
and email account, he gets the "there was an error signing you on - you need
to go to the Passport site and correct the error" message. he does so, signs
on to the passport site, gets a window about downloading MSN messenger,
which he doesn't want or need, closes the window down, tries to log onto
messenger, gets the same window, and so on in an endless loop.
I can sign on to messenger using his credentials on my machine without any
problem.
Can anyone tell me what is going on?
BTW I use Live Messenger on Vista, he is still on Windows Messenger if that
makes any difference....

Thanks!