There are different ways of installing multiple operating systems on a single machine. One way is to use a virtualization software. One can have a host operating system running multiple guest operating system as virtual machines. For ex. Windows XP can be running Ubuntu, Windows 7 as virtual machines. This will effectively give access to multiple operating systems with out an actual reboot of entire machine.
There are number of commercial as well as free virtualization software’s available (Extensive list), but my favorite is Virtual Box from sun micro-systems. VirtualBox is a powerful virtualization tool available for free to download and easy to use.
To install Ubuntu as a guest operating system on Windows XP one has to first download and install VirtualBox on windows. Then one has to download Ubuntu ISO in to a hard disk or burn into a CD. One’s VirtualBox is installed, using virtual machine wizard one can install Ubuntu on the windows. Installation and configuration of new OS is quite self explanatory.