When I last checked Virtualbox did not update their repositories for Hardy Heron, its not a problem, their is a hardy package without a repository. I have created a howto last year on howto setup VirtualBox completely and to seamlessly integrate windows xp into your ubuntu desktop here.
So lets install the brand new virtualbox package for Hardy Heron...
Lets Install the essential build utilities so the vbox kernel module builds.
Click for the Full Story
http://www.ubuntu-unleashed.com/2008/04/howto-install-virtualbox-in-hard...