Kevin Farnham describes at LinuxDevcenter how to setup a laptop to dual-boot Ubuntu and that other Operating System.
Enjoy the describtion here
I started with my aging Toshiba laptop (which had about 90 percent of its 30GB disk filled) and, without losing any data, turned it into a dual-boot XP/Ubuntu Linux system with a shared partition where many of my user files were accessible whether I was using XP or Ubuntu. This made the laptop much more versatile, which is ideal for a developer who works in Linux but must also work in Windows for certain applications or for Windows-based development. A few months later, the monitor on that machine gave out. I bought a new HP notebook and transformed it into a dual-boot XP/Ubuntu Linux system right away.
This article describes the steps I took to complete the dual-boot conversions.