Remote Assistance/Screen Sharing from Mac OS X to Windows XP analogous to iChat's Screen Sharing To help and teach older relative of mine who is on Windows XP I would like to have solution comparable to iChat's Screen Sharing.

Which means I can ask them for sharing their desktop with me (or get invitation from them) and to get full control over their computer with simultaneous audio chat while they can see what is on the screen and take control when they want/need.

P.S. As far as I know solution like RDC from Microsoft will let me to control remote Windows XP computer but will lock the remote user off the machine at the same time.
About the only thing I've found so far that is free and works well is logmein.com.  Another option is to use Skype but you'll never actually have control of the system.