If you want to dual boot Vista and XP, or just dump Vista and use XP alone, there are two major steps.
First, make sure all the needed XP drivers (chipset, networking, display, sound, etc.) are available for your PC from the PC manufacturer's web site. When/if you find them you may as well download them to another computer or removable media so that they will be readily available.
Second, assuming success in finding drivers, buy a legitimate Windows XP CD.
There are some minor steps I'd recommend also. For example, if your only way of reinstalling the Vista you already paid for is a Recovery Partition, make a set of Recovery DVDs/CDs while you still can. Even if you are going to dual boot, make sure your data is backed up in case anything goes wrong during the XP install. Actually, I recommend these steps independently of changing or adding an OS.