VirtualBox is an application that allows you to have several virtual machines inside your operative system.
Unfortunately the VirtualBox version 2.1.2 still isn’t on Ubuntu repository, but if you go to Download VirtualBox for Linux Hosts there you find a version that will run on Ubuntu!
Installing and running is really straight forward, but I had a problem when I tried to start a virtual machine:
VirtualBox can’t operate in VMX root mode. Please disable the KVM kernel extension, recompile your kernel and reboot (VERR_VMX_IN_VMX_ROOT_MODE).
To fix this I’ve just followed the indications in VirtualBox can’t operate in VMX root mode.
Here is VirtualBox 2.1.2 running Windows XP inside Ubuntu in Seamless mode