Multibooting is the installation of two or more operating system in a single computer. To dual boot XP and Linux is to install both operating system on the same hard drive. 

When you turn on the power and your computer starts to boot, it presents you with a menu and allows you to select which one of the installed operating system is to be booted up.

I'm happy with my Windows, why would I need Linux for?

Linux is fast becoming popular, and chances are you'll encounter one of their many flavors or distributions (called distros). It would be very advantageous for you if you have some experience with Linux so you'll be ready when you see one.

Linux is now easy to install and use rivalling Microsoft Windows in its user interface.

But the main reason I'm recommending multibooting is its security and backup implications. When your Windows installation is rendered useless by viruses, malwares, or rootkits, you can boot with the other operating system and do the cleaning and fixing. You'll have full access to all your Windows files. In the worst case scenario that you need to reformat your Windows installation, you can first copy your important documents before re-installing Windows.

My Windows XP installation got infected with a virus, my hard disk is infected, and since Linux is on the same hard disk on the same computer, isn't my Linux installation also already infected?

No. Definitely NOT.

When you were using Windows and got infected with a virus, Windows, and the Windows virus has no access to your Linux files since Linux is using a different file system. As a matter of fact, when you use your Windows installation, you can't see and access your Linux files although they are on the same hard disk. They are hidden from Windows.

I'll assume you already have Windows XP installed and running. If not, install Windows XP first before proceeding to install Linux.

Do not install a Linux distribution first and then install Windows XP later. It will make things more complicated. Windows XP needs to be in the first partition of the first hard drive. If you have installed Windows XP and you were able to boot it, then it is already located in the first partition of the first hard drive of your PC.

Download Ubuntu Linux at http://www.ubuntu.com/getubuntu/download

Install Ubuntu Linux. Run the installer and just follow the instructions on the screen.

What's next? Nothing. You're done. Reboot your PC and it should dual boot XP with Ubuntu Linux.

HOW TO RUN XP, VISTA AND LINUX ON THE SAME DRIVE (TRIPPLE BOOT!) - I took the decision to run more than operating system on my computer when Windows Vista made its debut a couple of months back. First, I wanted to run both Windows Vista and XP on the same machine so that I continue using my XP without interruption, but at the same time familiarizing myself with Vista. Later on, after the release of Ubuntu 8.04, I made the decision to run all three operating systems on the on laptop so that I get to enjoy the relative benefits of all the operating systems.

Install Ubuntu as a dual-boot with Windows without partitioning: How to Install Ubuntu without Any Partitioning - You may install Ubuntu operating system on your computer without any partitioning of your hard disk. These is a way by which you can install Ubuntu as a dual-boot with Windows without any partitioning. If you install Ubuntu in this way, you don't have to worry about partitioning. Many Windows user fear to partition their hard disk but wish to use Ubuntu. But they fear to partition and data loss.

Windows and Linux Multiboot Flash Drive - "Getting my Linux CD iso's to work (Knoppix etc) was fairly straightforward...but I wanted a live Windows system like the "Ultimate Boot CD for Windows" on there as well." Working in the IT industry, I am sometimes asked to recover some fairly major breakdowns and perform specialised tests, and I thought it would be useful to put some of my "live CD" tools onto a bootable flash drive. I thought it would be a simple job, and looked on the internet to find out the best way of doing it.

How to run Run Multiple OSs on One PC (or Mac) - You don't need separate computers to run different operating systems. Sure, Windows is the most common OS running on the millions of PCs in daily use, but that doesn't mean you're stuck with using only XP or Vista on your PC, or with using only OS X on a Mac.