http://wiki.onmac.net/index.php/HOWTO

Code:
 Introduction

Installing XP on an Intel iMac is a two step process.

    * Creating the installation media
    * Running the installer. 

The process is 95% automated. There are two sections below, detailing each one of the two steps highlighted above.

In each section there are three subsections:

    * What you'll need (Lists the requirements (hardware/software/time) in order to complete the task.)
    * Quick instructions (No-nonsense instructions for those "in the know". Assumes an advanced level of knowledge.)
    * Step-by-step instructions (Can get pretty dull at times, but are quite helpful if you want to avoid making any mistakes and regretting it later.) 

Choose one set of instructions or choose both and cross-reference between them. The advanced instructions give a good overview of how far you've gone, and you can drill into the step-by-step instructions anytime you have doubts.

So, without further ado...
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Preparing the XP installation CD

A custom slipstreamed CD is required to install XP on a Mac.
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What You'll Need

    * An original XP PRO SP2 CDROM (It doesn't have to be bootable, but it should have a i386 directory in the root.)
    * The winxponmac0.1.zip file
    * Nero Burning ROM
    * A blank CD (Preferrably a CD-RW, just in case it doesn't work first time)
    * A PC with XP already installed.
    * 20-30 minutes
The Bootloader File Download