Vista - Xbox Live Messenger Connection Issues

I was having trouble connecting to Messenger via Xbox Live.

When I selected the "Messenger" (M) icon on the chat-pad I was prompted for
a password and the sign-in process would begin.

I'd get knocked off of any other IM sessions on desktop machines with that
account - but it'd never finish signing in or display my buddy list - even
if left alone for hours.

If I signed back in on a PC, the sign-in process on Xbox Live would be
aborted.

Xbox Live Support was unable to identify the cause of the problem and
directed me to call MSN Messenger Support (though they didn't explain how to
do this or provide a number).

Eventually I decided to see if the number of contacts were too many. After
reducing my list to around 330 buddies, I still had the problem.

I used Messenger Plus! (http://www.msgplus.net - NOTE: Funded with adware,
pay attention during install if you wish to opt-out) and it's "contact list
clean-up function" to monitor my IM contacts over the span of about a month
and see who wasn't really using Live Messenger and, after exporting my
contact list, deleted 50 or so at a time until I was finally able to
sign-in.

The magic number seemed to be around 100 contacts (about the number of
contacts from my list that had been online sometime during the day
yesterday).

So if you're having trouble signing in to Xbox Live Messenger, but you
aren't receiving any error messages, try backing up your contact list and
cutting it down to about 100 or so.