Just found a rather insidious bug on Jenny's laptop. She upgraded to MSN Messenger 6.1 build 203 and then couldn't sign-in. Well, she would be signed-in for about half a second and then she would get automatically sign-ed out again. Weird. I could sign-in without any problems with my account but her account would fail. A quick reinstall didn't solve the problem.
I searched around the Internet and didn't find much help. I uninstalled MSN Messenger, deleted all the MSN data and I even trawled the Windows registry killing MSN entries. Then I reinstalled and we got the exact same behaviour. I could sign-in but Jenny got kicked out after half a second.
I finally recalled that a prime suspect in any weirdness like this is anti-virus software. She runs Norton Antivirus 2004 and sure enough I found it had a feature for protecting MSN Messenger. I turned this off and Jenny was able to connect ok. After hitting, the "Configure New User" button in Norton, I was able to turn the feature back on. Seems to be working now. It's nasty finding stuff like this so I hope this entry helps someone else.