Microsoft’s released a couple of client updates recently.
There’s a new client release: 4.0.7577.4384
2832123 You cannot start Lync 2010 after you install the March 2013 cumulative update for Lync 2010 in Windows XP Pro or Windows Server 2003
“This cumulative update improves the reliability, stability, and performance of the Lync 2010 Attendee”. This takes the attendee to 4.0.7577.4382