The first step to reverting to XP is to save your Vista data, the subtleties of which I'll get into later. After backing up the pertinent data, I used Acronis's True Image to make a backup of the entire Vista partition in case I forgot to copy something. This little fail-safe procedure has saved my tukus on more than one occasion.
Next, move your program data (your documents, images, and other files) to a safe location--that is, to a drive that won't be overwritten by your new XP installation.
What about applications? Alas, the tricks I describe below can't be used for migrating apps from Vista to XP. That step you must take after XP is installed, and, sadly, you'll have to reinstall your programs one-by-one to be certain they'll work right.
When you're ready to do it (not yet!), moving program data to Windows XP is easy enough. Instead of XP's My Documents, Vista stores data in a User folder that will bear the name of the current user and contains subfolders for documents, pictures, videos, contacts, and so on. Vista allows you to relocate these folders by simply dragging them to a new location. So, before you overwrite Vista with XP, do the following:
1.  Copy or relocate (right-button drag and select Move) your User folder or folders to a partition or drive that won't be overwritten--for example, the D: drive. Keep in mind that every user on the computer will have their own folder, so you need to relocate or copy all of them. And don't forget the Public folder.
2.  If you've been using programs that allow it, export data to a safe place for later import  when you've reinstalled XP. More on this later.
3.  Once you've installed XP, either drag the user folders back to XP's My Documents, point My Documents to the folders' new location using Properties, Target, Move, or simply access the data where it resides.
4.  Import your exported data back into your third-party apps after you've reinstalled them.
There is data hidden in your User folders that you may or may not need (the default e-mail repository, for example, is here). To see it, open your User folder, select Folder and Search Options from the Organize menu, select the View tab, and make sure Show hidden files and folders is selected.
Close the dialog box, and you'll see a folder called AppData; it can't be moved like the other User subfolders. Open it, browse it, and if you see anything you might need, copy it to a safe place. (You could skip this step and grab it later if you discover you need anything, since it will have been copied when you made your backup image. But copying it this way makes it easier to access.)
Don't install XP yet! You're nearly there, but a few more details must be dealt with.