To enable guest additions in X either run /usr/share/virtualbox/x11config.pl or
edit your xorg.conf manually. The video driver has to be set to "vboxvideo" and
the mouse driver to "vboxmouse". Additionally the options "alwayscore" and
"sendcoreevents" should be removed from the mouse section.

 -- Michael Meskes <meskes@debian.org>  Mon, 19 Nov 2007 11:17:46 +0100
