Same issue as dj_bridges, with one exception. When the KDE taskbar panel is hidden, the xp guest taskbar will not respond to mouse clicks, if in full screen mode. When the KDE taskbar panel is set to always visable, the xp guest taskbar works normally in full screen mode.
Sorry for resurrecting this old post, but it seemed silly to start another thread for the same problem.
Host PCLinuxOS KDE 2011.6 fully updated, Guest Windows XP Pro SP3 Guest Additions installed, Virtualbox 4.0.12 r72916.