If you disable Plus!, does it still freeze? I'm not sure if it's caused by Plus!, if it is, this thread belongs to the Bug Report forums. This used to happen to me after I sent a screenshot using the Screenshot Sender 4 script. If you have any scripts running, disable them and try again.
Also, I recommend updating to Windows Live Messenger 8.5, just download the update from
http://messenger.live.com/.
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