OEM versions of Windows XP:
-- cannot upgrade over an existing Windows installation.
-- must be installed "clean" on a freshly reformatted drive or partition.
-- cannot be transferred to a different computer or if motherboard is upgraded.
-- the license cannot be sold or transferred to another user.
-- are not eligible for free Microsoft technical support.
-- any problems whatsoever with the installation CD or Product Key.
-- is not eligible for Microsoft support....you have to deal with the "seller".
-- cost less than "retail versions" due to the above limitations/risks.
Best Advice: Purchase a "Retail Version" of Windows XP!
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Differences between XP Home & XP Pro:
http://www.winsupersite.com/showcase/windowsxp_home_pro.asp
Note: Google is your friend. Just type in your question like you did here and you'll get tons of hits.