Microsoft Windows XP comes in two flavours, the standard 32-bit operating system we're all familiar with and a 64-bit version called WindowsXP x64 Edition. The availability of 64-bit CPUs has made it possible to run a fully 64-bit operating system and processor, but it's not always so convenient when it comes to software applications or games. Our solution is to install both OSs on one PC, allowing you to switch between 64-bit and 32-bit! - Version 1.0.0
How dual-booting Windows XP works
Here's the rundown on multiple Windows installations: Windows XP uses the BOOT.INI file to identify which versions of Windows reside on which disk partitions on the system. BOOT.INI (as well as the NTLDR and NTDETECT files) must always be present on the first bootable partition (the primary partition of whichever hard disk the BIOS attempts to boot from first), usually the C:\ drive. The actual files and directories that make up each Windows operating system can be located on any drive or partition as long as an entry for them exists in the BOOT.INI file.
As long as each new Windows operating system you add can create or make changes to BOOT.ini, the NTLDR file which actually starts the Windows XP boot process will know where the system files for each OS are located, allowing those operating systems to load. Because of this, it's important to add operating systems onto your system by order of date when you are creating a dual-boot. Older operating systems should be installed before later ones, because earlier versions of NTLDR may not be able to accommodate the needs of later Windows operating systems. Pre-Win2k, NTLDR was not used at all.
This system makes multiple installs of any recent Microsoft operating system extremely easy, provided you do not mess with the files on the original partition. The boot loader for XP will happily accommodate pointers to any number of Microsoft operating systems on different drives and partitions, giving you a choice each time you start your computer.
In order for this guide to work, we've got to make a couple of assumptions: number one, that you have a 64-bit capable processor (either AMD Athlon 64, Opteron or EMT-64 enabled Intel Pentium 4/D/EE) and a working 32-bit installation of Windows XP (though in practice, Windows 2000 or 98 will work just fine too), and number two, that you have a free partition on one of your physical hard drives which you can use to host the operating system files for the 64-bit version of Windows XP.
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