Mircosoft promises to give away Windows 8 to their new customers at $15 (INR 699). To avail this offer you would have to buy a Windows 7 PC between 2nd June 2012 and 31 January 2013. The last day to register and order your Windows 8 upgrade is 28 February 2013.
You would have to provide your contact details, retailer information along with the current Windows 7 product key.
Visit the offer's website to help yourself with FAQ's