how do i unstall vista on my pc and put xp back on?

Answer 1

If XP was on the computer and you upgraded to Vista, you can try uninstalling Vista and reinstalling XP.

However, if the computer came with Vista loaded, and you have the XP CD from a different computer, it will not fly.

When you install or activate a Microsoft program, MS takes the product key of the program and mixes it with information about the computer on which you are installing. These numbers are then used to generate a third number called the installation ID. Obviously, they will not match a different computer. This is one way MS is preventing piracy.

Microsoft Activation/Piracy Prevention – full explanation at source link.
Rather long article, but will answer all your questions about activation and what is or is not allowed.
Source: http://www.microsoft.com/piracy/activation_faq.mspx

How does Microsoft Product Activation work?
Source: http://www.microsoft.com/piracy/activation_faq.mspx

Product Activation works by validating that the software's product key, required as part of product installation, has not been used on more PCs than is allowed by the software's end user license agreement (EULA). In general, Windows XP can be installed on one PC and Office 2003 or Office XP can be installed on one PC and the laptop computer used by the user of the one PC. (For specifics, please see the EULA accompanying your product.) Product key information, in the form of the product ID, is sent along with a "hardware hash" (a non-unique number generated from the PC's hardware configuration) to Microsoft's activation system during activation. In Windows XP SP1, the product key itself is sent in addition to the product ID. Activation is completed either directly via the Internet or by a telephone call to a customer service representative. Activations on the same PC using the same product key are unlimited. Product Activation discourages piracy by limiting the number of times a product key can be activated on different PCs.
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How will Microsoft Product Activation help thwart piracy?
Source: http://www.microsoft.com/piracy/activation_faq.mspx

Product Activation will help reduce casual copying by ensuring that the copy of the software product being installed is legal and has been installed on a PC in compliance with the End User License Agreement (EULA). Installations beyond those allowed in the license agreement will fail to activate.