Most laptops, which are already installed, come with a Vista back up CD. If you have that, you can install the OS. If you do not have that, you do need to buy it again. Unfortunatly there is no other way to go.
http://www.microsoft.com/windows/products/windowsvista/editions/homebasic/default.mspx.
That link will take you to Vistas site, and it will expain it more to you.
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Election Information
George Carlin FTW