first : after i close the compiz in system --> admin --> appearance
it can not be startup again and display desktop effect could not be enable
solution: i reconfig the xorg and choosing option ---> session ---> gnome at login page
and compiz can run #vamp#
second: virtualbox could not be start by click the icon located at applications--->accessories
when i run it with terminal,i got "Could not find VirtualBox installation. Please reinstall."
then i remove virtualbox and delete the .Virtualbox folder at home direction
solution i got a non-free version virtualbox from sun http://dlc.sun.com/virtualbox/vboxdownload.html#linux
and install Ubuntu 8.04 (Hardy Heron) version
finally it works
