For any dual boot system to work you must have a spare partition of your disk available for the install. This keeps the two instances completely separate from each other. For some, you may only have only one disk partition in your system or you may have multiple disk/partitions which are being used. The thought of recreating your volume, formatting can be daunting if not unnecessary. Fortunately, whichever operating system you use, you can resize your existing partitions(s) to free up some space for a new one.

If you already have a spare partition then you can skip this section, otherwise please read on.

Resizing you partition(s)

This section assumes you don’t have a spare partition to install your secondary Operating System or you have a spare partition but it might not be large enough. Below is a list of the recommended disk/partition sizes:

• XP Home and Professional – Requires at least 1.5GB but allow for at least 5GB
• Vista Home Basic - 20 GB hard drive with at least 15 GB of available space
• Home Premium / Business / Ultimate - 40 GB hard drive with at least 15 GB of available space
• Windows 7 RC - 6 GB available disk space (32-bit) / 20 GB (64-bit)

(Source – Microsoft)

If you have XP installed, you will have to use a either the DiskPart utility which is on the XP install disk or a third party program such as GParted. Either will do the trick.

If you have Windows Vista installed then your in luck. Vista comes with its own disk partitioning utility in Disk Manager which is just a GUI version of DiskPart and provides an easy method of resizing your partitions.

Click on Start> Computer> Manage

In computer manager select Disk Management.

Right click on the primary partition you wish to reduce and select “Shrink Volume”

Vista will now shrink the partition by the size you specified giving you a spare partition to install your alternative OS. When you return the Disk manager you will see your new partition which you can format.

You can use this utility to readjust existing partitions if need be.

Now you have created a new partition, you can now proceed to load the secondary OS.

Installing your secondary Operating System

Windows assumes you are installing the system in their chorological release order, so creating a dual boot system from this perspective isn’t a problem. However, when this procedure is reversed, the process is a little more complicated.

If you have XP already installed and whish to install Vista/Windows 7

This is simple. Simply boot from your Windows Vista or Windows 7 install disk and continue with the installation. When asked “where do you want to install Windows?” simply select the free partition or the “Unallocated space” and click on next. The install will then install Vista in this partition. When finished, XP from hereon will boot via Vista boot manager. You will be asked to select which Operating System to boot to, either an “earlier version of windows” in this case Windows XP or Windows Vista or Windows 7”.

If you have Vista or Windows 7 already installed and whish to install XP

This process is a bit more complicated. Firstly you need to boot from your Windows XP install CD and install. When asked where to install the Operating system, select your free partition or unallocated space. Complete the XP installation as normal. This process will override the Vista’s Boot Loader initiated from the boot sector. Therefore, the system will only boot into XP. Don’t panic, Vista is still on your system, you just need to modify how the system boots.  

Boot from your Windows Vista or Windows 7 DVD, and when prompted click on “Repair your Computer”. Select Install and then run “Start-up Repair”. This will recreate the boot manager you removed earlier but in doing so will replace the XP boot process. The boot loader knows nothing of the XP installation but again, the XP OS is still their but you need to add XP to the Boot manager.

Logon to Vista or Windows 7 as an Administrator and open a command window and enter the following commands:

bcdedit /create {ntldr} /d "Windows XP"

bcdedit /set {ntldr} device boot

bcdedit /set {ntldr} path \ntldr

bcdedit /displayorder {ntldr} /addlast

When done, close the window and reboot. Next time round, the system will ask which Operating System to boot to, either an “earlier version of windows” (in this case Windows XP) or Microsoft Windows Vista or Windows 7”.