Depending on your version of Vista, you'll need to purchase a new Windows XP SP2 disc, then research, find, download, and copy all the hardware drivers to a CD, then follow many of the steps listed in this long discussion about how to perform the downgrade.. (The discussion at the link below is primarily about HP computers but there are other topics about other brand names.. Unfortunately, you didn't provide us your info.)
http://forums.cnet.com/5208-12546_102-0.html?forumID=133&threadID=243153&messageID=2460401
Hope this helps.
Grif
Can Vista be removed from a computer recently purchased and replaced with Windows XP after reformating the harddrive?

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