Recently I have been involved in several cases involving users on Windows XP SP3 machines who have upgraded to the Communicator R2 client and do not have the Share Desktop feature available in Communicator R2. The whole “Share” option is completely missing from the right-click menu on a user contact!
In all cases installing MSXML 6.0 SP1 and restarting Communicator R2 has resolved the issue.
As per the Office Communications Server 2007 R2 Client System Requirements the minimum Operation System requirements for Windows XP is only SP2, but it appears the Share feature has a dependency on a more recent version of MSXML (or it is getting uninstalled during the upgrade process). Despite no mention of this prerequisite in the Communicator R2 Deployment Guide, Release Notes, or System Requirements, I found a TechNet article stating that indeed MSXML 6.0 is a requirement for Communicator R2.
Desktop Sharing is one the best new features of Communicator R2 (see my previous post: Communicator R2 Desktop Sharing Rocks!).