I like the closed source version of virtualbox. I just do. This is how to install it quickly and simply:

add this to sources.list

deb http://download.virtualbox.org/virtualbox/debian lenny non-free

run this to get the public key and install it:

wget -q http://download.virtualbox.org/virtualbox/debian/sun_vbox.asc -O- | sudo apt-key add -

then just install it

aptitude install virtualbox-2.1

this will automatically create the group “vboxusers” and add the user who installed it to the group. If you need to add others:

sudo adduser `[username]` vboxusers