Running Virtualbox in WUBI does not make sense because it will eat up your
space which is fixed. Virtualbox itself is not the worry. It is the Windows
XP vdi that will be large enough to hold a full Windows XP distribution.
Even if you set aside 20 GBs for Ubuntu's image in WUBI you could lose half
of that for XP's image file in VB. As the Ubuntu space diminishes
performance drops off significantly.

There are two possibilities for Virtualbox that do make sense. One would be
to set up Virtualbox in Windows and run Ubuntu in the VM. The other would be
to get rid of WUBI and set up a partition for Ubuntu. Install Ubuntu as a
dual boot operating system. You could then install Virtualbox in Ubuntu and
run Windows in a VM. Remember that you will need a Windows license to do
this legally. The first option has no such restriction.

Virtualbox is easy to set up in either Windows or Linux. It installs like
any other application. If you plan on using usb devices in Virtualbox, do
not instlal the OSE version in the repositories. Get the PUEL one from
Virtualbox website. It supports usb and is still free as in cost, but the
license is restricted. The only hitch in installing VB in Ubuntu is that you
have to edit the users file to make yourself a member of the vboxusers
group. This is done from the GUI as is everything else.