Apple Boot Camp lets you install Windows XP on a Macintosh Computer using the original Microsoft Windows XP installation CD with the product key. After installation, you can start up you Macintosh using either Windows XP or Mac OS X.
Here are the steps required to install Windows XP on a Mac:
Step 1: Download Mac OS X v10.4.6 (or later) and latest firmware update from Apple Download website.
Step 2: Run the Boot Camp Assistant to create a Macintosh Drivers CD or DVD.
Step 3: Create a new hard-disk partition [minimum size, 5GB] to make room for Windows XP. All the software and data on the original Mac partition stays intact.
Continue at source:
Boot Camp Tutorial