Virtualbox can run whole operating system in a window,but that do not have direct access to your computer hardware,Virtualbox can run all versions of Windows,Linux and various other systems.
To install virtualbox,you need to install following package by running following command in a terminal window first:
sudo apt-get update sudo apt-get install dkms
Then download .deb package for virtualbox install from the link:http://www.virtualbox.org/wiki/Linux_Downloads
Just double click on the package you downloaded,click Install Package in next window and install it by prompt.
After install finish,restart computer,then you can open it at Applications–>System Tools–>Sun Virtual Box,when you first open up you will see following figure,scroll down to bottom and select I agree
then create user name
you will see virtualbox work window:
click here for installing windows XP with virtualbox inside ubuntu