As Lee says you cant install XP "over the top of Vista" - the default option will be to install it on the same drive - which would give you a dual boot of "working Xp" or "broken Vista"...pointless. Best to format the whole thing but you will almost certaily need some drivers, most you will be able to get from your motherboard manufacturers site, or if its a badged PC like a Dell, from their site.
However, i strongly advise you to download Linux Mint and use that instead, much less hassle. Boot from the DVD and and see if it picks up all your drivers, it usually does. If you like it, install it after formatting the drive. Its a FAR easier install than XP. And you can select extra programs you may need simply by selecting "software manager" and once you install "wine" you can install windows programs too. Really simply. Go on, go for it.
(ubuntu is good too, but linux mint defaults are more helpful to newcomers)
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