To make a dual boot with Windows Vista over XP, repartition the free space on the C drive, install Windows XP, and restart the computer. Learn how to free up space on a computer drive in order to accomplish a dual boot with instructions from a network engineer and IT specialist in this free video on Microsoft Windows Vista.
Vista has a free disk partitioning utility, that was limited in prior versions of Windows, but is now a little bit more robust, and allows you to resize an active partition, say your C drive, that Windows Vista is already installed on, and cut down the partition down into a smaller portion, to allow free space. Once you have free space, you can then repartition that free space into whatever you want. In this case, we want to be able to dual boot, so once that free space is partitioned, then you can install Windows XP over it. How to do that is very simple. You'll go into your Start menu, you right click on My Computer, go to Manage, and then it will give you a graphical representation of how much space is available, and which partitions you have. In my case, I already have an extra partition, called the D drive, on my computer, and I'll show you that now. Click on Disk Management under storage, when you right click on My Computer, and select Manage, and then it will bring up a list of active partitions and free space, if you have any. In my case, I don't have any, because I'm a geek, so let's take a look here. I have a C drive. I have a D drive, which is on the same physical disk. It's represented here by disk zero, so it shows C, and then it shows D, so basically, if you wanted to take your C drive, which is what most people have, you'd right click on it, and say Shrink Volume, and that will pull it down, and then after it's shrunk, you'll then see a free space here, that will, and then you can right click on that free space, and then partition it. Once it's partitioned, the Windows XP setup will see it, and you can then install Windows XP. Once that is installed, you'll then be able to dual, simply by restarting your computer, and selecting XP or Vista, when you restart.