Sounds like you have a custom action set on the event of your contact logging in... check the Get Info window for yourself on your contact list, in the Events tab.
I don't see myself on my contact list. :-/ When I look at the events tab in preferences, for the option "You connect" all I have set is "Play the sound Welcome"
This is more perplexing than annoying, but still I'd like to solve it. :-)
I did that right after posting, hadn't gotten a chance to edit. No events at all are set.
I did discover that it's not jabber but the yahoo account that's causing it. I forgot I I had that account logging in. I have no contacts but only use it for mail checking. I've checked event setting for that user account and nothing is set.