It's my thought that there must be some sort of setting in IE7 when you've upgraded MSN to a later build (only path that makes sense).
It's my thoughts that there must be a setting in either of the two products that reads a name and tries to message it or something similar, but in either case, as you've said it's a smalls ection of users, so chances are theres a little more that isn't being provided to you (either with their environment or what they're doing for this behavior to happen)
