14.04 virtualbox guest OS(xp) key conflict

Hello,
After install 14.04 yesterday, I found that the keys are conflicted between Virtualbox and Ubuntu.
when I press Super(Win) key in guest os(XP), both Dash menu and Windows Start menu pop up,
if I press Alt+Tab key in guest os, Ubuntu swithcher pop up.
I can confirm that in Ubuntu 12.04, not this key problem.
Could you help on this ?
Thanks!
chenzero