You will have to partition, since you are at the moment using all your drive for Vista, so if you now tried to install XP, it'd wipe your Vista installation, most likely with all your files, and put XP on.
To dual boot the easy way, you will have to format, and then install both again, setting up a partition on the XP setup, or whichever you decide to install first.
On the other hand, you can use a program called Partition Magic to partition half the drive as it is now within Windows (rather easy to do actually), with space you aren't using, and then on that empty partition, install XP with the setup disc once you restart the PC, with the virtual drive its created.