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If you are getting Window for commercial use then I would recommend you to purchase it from any retailer. However if you want Windows for your personal use, you can either purchase it or download it from from the Internet. Best known site is torrentz. Happy Downloading :)
Guys please do not use those links as they may contain malicious programs which can steal your personal information and can cause harm to your computer. If at all possible, try to purchase a branded computer that comes with pre-loaded Windows or purchase the software from authorized dealer or switch onto Linux. Using pirated software is illegal.