How can I install Windows XP on a computer which already has Windows Vista installed?

There are several ways, both easy and difficult, to install Windows XP or a lower version of Windows on a system with a higher version of Windows on it. Firstly, you will need an additional partition, where Windows XP will be installed as the existing drive C already contains Windows Vista. Using Vista, you can simply resize your existing partition and create a new one from the disk space that was freed up. Use the Disk Management utility of Windows Vista to help create the new partition. Once done, you will need to boot through your Windows XP disc and install Windows XP on this new partition. After this procedure, you will no longer be able to boot Windows Vista as Windows XP has overwritten the boot loader. This is because a lower version of Windows does not recognize the higher version and thus will not make the appropriate entries in the boot loader to accommodate both operating systems. To trick the boot loader and add the Vista entries along with XP, you can use a free utility called VistaBootPro. This utility will do the needful for you. As I cannot explain the whole procedure step-by-step in this section, you can follow the link http://www.howtogeek.com/howto/windows-vista/installwindows-xp-on-your-pre-installedwindows-vista-computer/ , which will help you. Note that this procedure is recommended for those who know how to trouble shoot operating system errors.

VistaBootPro is a free utility that helps manage and restore the boot loader for Windows Vista.

How to install Windows XP without anyone changing the existing OS

Though you can install Windows XP or any other operating system as a second OS on your computer, you still cannot protect your fi les from another person accessing the fi les on your primary OS partition. There are ways that he/she can access your important fi les. If you want to protect your fi les and settings from the other user, I would advise you to create a user profile which is a standard user instead of a user with Admin rights. This will create a different account for the user and limit the areas he/she can access. The only other way you can keep the other user from getting into your data is to install another hard drive with an operating system on it for the other person to use while disabling your main hard drive from the system BIOS. You would have to go through the trouble of enabling and disabling the drives from the BIOS every time you or the other person uses the computer. Alternatively, use encryption software such as TrueCrypt.