Prior to an OS down grade, you should note what was included with the PC restore wise, with mine I got an Upgrade Vista DVD and a Recovery DVD from Gateway which is the Operating System Only, niether of these include the full driver software for my computer just generic ones or none at all.
The first thing I did was make the DVD's of the Full Factory Install and the Drivers and Software with the Recovery program included with my computer. In Device Manager I made a note of the names of the hardware in the computer, you can also use the recommended 3rd party software, I used SIW since it doesn't require install and seemed to work well to identify my stuff.
Rather then format and install only XP I decided to set up dual boot so I shrunk the drive within Vista and created space and a drive letter to install XP on. I tracked down the driver software for my video, modem, LAN, Intel chipset, sound and put them in a folder in the Recovery Partition on the hard drive and to a CD for install after the install of XP. I put the XP CD in and rebooted and entered BOIS and changed the RAID setting on the hard drive to IDE under the Advanced tab, this makes it so you don't have to load on SATA drivers during install if you do not have them or a Floppy Drive in general, after saving the settings and exiting I pressed any key to boot from the CD when the screen came up. During the install you will pick the Drive you made in Vista to install XP on. After the install the computer will only boot to XP because it changes the Vista boot loader so unless you want the option to boot to Vista you can leave it that way, it also changes the factory installed boot so you won't be able to use F11 to restore the computer so it is important to make the factory install DVD just in case you need to go all the way back.
I later decided to hook a temporary floppy to the computer and installed the SATA drivers, entered BOIS and set the IDE back to RAID, did a repair with XP and it loaded in the drivers and I didn't loose any information, I found that the computer really wasn't that much faster least that I could tell with the SATA over the IDE so was probably a waste of my time.
Upside to this way - if you do decide later down the road you want to go back to Vista it's a matter of restoring the boot loader with the Vista DVD, moving the things in XP that you did over to the Vista Partition and expanding the drive, while using XP you have access to all the things you did in Vista Partition. Down side, my shrink gave me 80G of the hard drive - for some that may not be enough but keep in mind large things can be stored on the Vista Partition if more space is needed. Also altho my install went smoothly I did have problems installing the modem in XP being on broadband I have decided to let that go and lastly some programs altho I did not have the problem with any of mine will not install on any other drive letter but C.
The post about Googling how to dual boot vista xp is where I found the directions to do this and I had printed them out to follow step by step.
I have found that MS slightly missed the mark with this operating system since I believe that it is slightly more intense on your hardware then it should be but I have also found that vendors are using it as a way to get you to buy a new one of what isn't compatible with Vista hence making you lash back at MS, like my older HP all in one, HP said they gave the generic drivers to MS and if I want full control over the all in one with thier software they gave me a link to what they now offer that is compatible with Vista, I actually have come to prefer the Vista supplied ones as the CD to load on the printer in XP was mainly bloat ware anyway, now I can use programs already on my computer for the scanner etc.
Since everyone was aware this operating system was coming for 5 yrs now it would seem that vendors have choosen to move on with thier new product drivers rather then waste man hrs on supporting the older ones.
Northlite