Anyone looking to virtualize guest operating systems on an Ubuntu desktop really can’t go wrong with VirtualBox. For personal use you can either install the Open Source or the more fully featured Full edition. I will be installing the later as it has support for USB and some 3D acceleration (among its many other features).
Installation in Lucid is as follows:
Open the Terminal and edit your /etc/apt/sources.list adding this line:
deb http://download.virtualbox.org/virtualbox/debian lucid contrib
Download and register the public key:
wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -
Install VirtualBox:
sudo apt-get update sudo apt-get install virtualbox-4.0
Click Applications, System Tools and Oracle VM VirtualBox to run the application.
Here you can see VirtualBox running with a Windows 7 virtual machine that I have already installed.
Source:
Michael Lane's Technology Blog 
what do you mean “edit /etc/apt/sources.list”
I use nano to edit files using the terminal. The following command will let you edit the file /etc/apt/sources.list:
sudo nano /etc/apt/sources.list
If you do not have nano installed:
sudo apt-get update
sudo apt-get install nano
Hope that helps!