Virtualization via Oracle VM VirtualBox can allow you to run multiple Operating Systems Simultaneously on the same machine.
Not all users will have Windows Ultimate as their OS which will allow you to run Windows XP mode (Virtual Windows XP) which can allow for legacy software to be run.
VirtualBox is a great solution allowing you to run Operating Systems such as XP, Vista and even some of the popular linux operating system distributions.
These could be Arch Linux,Debian,Fedora,Red Hat,Linux Mint and Ubuntu as well as many others. It all depends on your needs which operating system you may wish to run inside Virtualbox!
Virtualization also aids testing for new operating systems such as the beta release of Window 8 so you can see if you would like to upgrade your main OS.