You would have to call Microsoft or contact there support team if you have proof of purchase they can give u product key. i would also think about contacting the original manufacturer of the computer as they could provide u if you have proof of purchase. if not you could always purchase a windows vista or windows 7 operating system. microsofts number is 800-642-7676 and there Hours of Operation: Mon-Fri: 5am-9pm PT
Sat-Sun: 6am-3pm PT