"How do I install a dual-boot with Windows XP on my new Windows Vista computer?" The answer is that it's not that difficult, it's just very time consuming, and you need to own a copy of Windows XP.
Note that you should not attempt this if you aren't ready to troubleshoot any problems that might occur.
The first issue we encounter is that computers with pre-installed operating systems take up the entire drive. Luckily Microsoft included the Shrink volume feature in Vista, so we can easily shrink the Vista partition down to make room for XP.
Open the Computer Management panel, which you can find under Administrative tools or by right-clicking the Computer item in the start menu and choosing Manage. Find the Disk Management item in the list and select that.
Now we'll shrink our volume down by right-clicking on the main hard drive and choosing Shrink Volume.
Now you can choose the size that you want to shrink, which really means you are choosing the size that you want your XP partition to be. Whatever you do, don't just use the default. I chose roughly 10gb by entering 10000 into the amount.
The next step might be confusing, because we need to change the cd-rom drive that's invariably taking up D: at the moment, because we want to use D: for the Windows XP partition, but it's already taken by the cd-rom drive. If you skip this step than XP will install onto the E: drive, which isn't the end of the world, but it's not quite as tidy.
Right-click on the cd-rom drive in the list and choose Change Drive Letter and Paths from the menu.
Now we'll change the CD drive to use E: by selecting that in the drop-down.
Now we can create a new partition for XP to live on and make sure that the drive letter is set the way we want. If you do not create a partition now the XP install will do so automatically, but it's easier and cleaner to do it this way.
Right-click on the Unallocated free space area and then select New Simple Volume from the menu.
Follow through the wizard and select whatever options you'd like, making sure to use D: as the drive letter.
Now you will need to close out of disk management and reboot your computer. This is because we can't do the next step until we reboot. (you can try, but it won't work)
So we've come back from rebooting… open up Computer from the start menu and then right-click on the D: drive and select properties. Give your partition a meaningful name like "XP". It would be wise to name the C: drive to "Vista" at this point as well.
Now you'll want to pop your XP cd into the drive and boot off it. You may have to configure your BIOS to enable booting off the CD drive, or if your computer says something like "Hit Esc for boot menu" you might want to use that.
Once you come to the screen where you can choose the partition to install on, then choose either the unpartitioned space or the new partition you created. Whatever you do, don't try and install onto your Vista partition! See how much cleaner it is now that we've labeled each partition distinctly?